A highly sensitive lifetime-based luminescent manometer and bi-functional pressure–temperature sensor based on a spectral shift of the R-line of Mn⁴⁺ in K₂Ge₄O₉

Abstract

Although lifetime-based luminescent manometers exploiting the luminescence kinetics of Mn⁴⁺ ions exhibit remarkably attractive manometric performance, the number of studies reported in the literature devoted to this area is relatively small. Given that this type of manometer can exhibit very high sensitivity over a limited pressure range, in-depth studies on the correlation of the structure with the manometric properties of such pressure gauges are required to enable the design of manometers with predefined sensing performance. In response to these requirements, the present work investigates the spectroscopic properties of a new promising inorganic phosphor, i.e. K₂Ge₄O₉:Mn⁴⁺, measured as a function of pressure and temperature. As is shown, the spectral shift of the Mn⁴⁺ R-line, the ratio of its emission intensity to phonon-progression bands, and the luminescence kinetics of the ²E state can be used for remote pressure readout, with sensitivities of 0.59 nm GPa⁻¹, S_R = 21.7%/Gpa and 12%/Gpa, respectively. Notably, the developed manometer shows the highest sensitivity in the lifetime-based mode reported so far for pressure values above 2 Gpa. Furthermore, the considerable thermal sensitivity of the narrow emission line of Mn⁴⁺ ions (R-line) in the K₂Ge₄O₉:0.1% Mn⁴⁺ material, combined with its minimal shift under pressure for pressures below 1 GPa allows for the utilization of this phosphor as a luminescent sensor capable of concurrently measuring both pressure and temperature.

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L. Marciniak

Lukasz Marciniak (prof. in physics) received his PhD and habilitation in Physics at the Institute of Low Temperature and Structure Research, Polish Academy of Sciences (ILTSR PAS), in 2014 and 2017, respectively. In 2022 he become a full professor. He is the co-author of more than 200 publications and 12 patents. His current research focuses on the luminescence properties of advanced multifunctional materials with a special emphasis on their application in remote sensing and imaging of physical and chemical parameters i.e. temperature (luminescent thermometer), pressure (luminescent manometry) and pH. He conducts interdisciplinary scientific research on the borders of physics, chemistry, chemical engineering, material engineering, and biology.

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Introduction

The utilization of phosphors for the development of luminescent sensors has facilitated the remote readout of physical and chemical quantities with a high spatial resolution in an electrically passive manner.^1–9 As demonstrated by research in luminescence thermometry, among various spectroscopic parameters, two sensing modes, i.e. ratiometric and based on luminescence kinetics (lifetimes) offer the highest potential for imaging of physical quantities.^10–12 However, in the ratiometric mode, where the relative intensity variation of two emission bands is analyzed, the dispersive dependence of the extinction coefficient of the medium containing the phosphor or lying in the optical path between the phosphor and the detector, under specific conditions as well as the optical detection system, can bias the intensity ratio, leading to errors in the pressure readouts.¹¹ Therefore, an effective strategy often proposed in the literature is the use of sensors involving luminescence kinetics as a metrological parameter for diverse sensing purposes.^1,10,13–15 However, despite its attractiveness, this approach is modestly employed in luminescence manometry.^16–20 Pressure applied to the phosphor can induce various effects altering the luminescence kinetics of the emitting state, such as (I) phase transitions, resulting in a significant change in the energy level structure of the system; (II) formation of crystal defects, which may result in energy migration to traps and defect states; and (III) shortening the interionic distances, leading to increased probabilities of interionic energy transfer, which may induce nonradiative depopulation of emitting levels due to cross-relaxation processes.^21–24 A key example is SrF₂:Yb³⁺,Er³⁺ where growing pressure activates cross-relaxation, shortening the lifetimes of the ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ions.¹⁷ Similar effects can be observed for other systems doped with lanthanide ions.^25–28 However, it should be noted that temperature elevation enhances the probability of depopulation of excited levels through multiphonon processes. Hence, temperature and pressure changes in the system typically manifest in a very similar way in relation to the luminescence kinetics of lanthanide ions.¹⁷ This not only introduces the possibility of deterioration in the reliability and credibility of sensing readouts but also hinders the measurement of pressure independently of temperature. Therefore, an alternative approach is necessary for the detection of pressure changes under extreme conditions. As a response to this demand lifetime-based luminescent manometers based on materials doped with transition metal (TM) ions have been recently proposed.^{18,19,29–32} Due to the unique electronic configuration of TM ions, their spectroscopic properties exhibit high sensitivity to the changes in the local environment of the ions. In general, the compression of the phosphor induces a change in the strength of the crystal field acting on these ions. This is particularly evident for ions with a 3d³ electronic configuration (e.g., Cr³⁺, Mn⁴⁺), where pressure growth boosts the crystal field strength, elevating, among others, the energy of the ⁴T₂ energy level. Due to the spin–orbit coupling between the ²E and ⁴T₂ levels, the energy increase of the ⁴T₂ level reduces the coupling strength, leading to the prolongation of the lifetime of the ²E level. The observed prolongation of the lifetime is a unique effect that distinguishes this type of luminescent manometer from other lifetime-based pressure gauges. Importantly, for most phosphors doped with Mn⁴⁺ ions, the high activation energy of nonradiative depopulation of the ²E level renders the luminescence kinetics associated with the ²E → ⁴A₂ electronic transition insensitive to temperature changes at room temperature. While the theoretical foundations of this solution seem particularly attractive from an application perspective, they have not been extensively explored in the literature to date.^18,19

Therefore, this study presents a comprehensive analysis of the spectroscopic properties of K₂Ge₄O₉:Mn⁴⁺ as a function of pressure and temperature to evaluate its application potential for remote pressure and temperature sensing. The performed analysis enables the selection of the optimal dopant concentration to obtain intense emission and a long lifetime of the ²E state. The high-pressure spectroscopy analysis indicates that the spectral and temporal luminescence properties of the K₂Ge₄O₉:Mn⁴⁺ material are significantly affected by pressure. Therefore, the spectral shift, luminescence intensity ratio, and lifetime-based approaches to luminescence manometry were investigated and their thermal sensitivities were analyzed. Moreover, utilizing the unprecedentedly large, temperature-induced spectral shift of the ultra-narrow emission band of Mn⁴⁺ (R-line), and its negligible pressure-dependence in the low-pressure range, we were able to develop a truly bi-functional manometer-thermometer operating under extreme conditions.

Experimental

K₂Ge₄O₉ powders doped with different concentrations of Mn⁴⁺ were obtained by using the solid-state synthesis method. Germanium oxide (GeO₂; 99.999% purity; from Sigma Aldrich) and manganese(II) chloride tetrahydrate (MnCl₂·4H₂O; 99% purity; from Sigma Aldrich) were used in stoichiometric amounts. On the other hand, potassium carbonate (K₂CO₃; 99.997% purity; from Alfa Aesar) was used in a 20% excess. The raw materials were ground in an agate mortar with hexane, transferred to porcelain crucibles and annealed at 1123 K for 6 hours under an air atmosphere. The obtained materials were then examined by X-ray powder diffraction (XRD) using a PANalytical X’Pert Pro diffractometer in Bragg–Brentano geometry equipped with an Anton Paar TCU1000 N temperature control unit using Ni-filtered Cu K α radiation (V = 40 kV, I = 30 mA). Measurements were performed in the range of 10–90°. Transmission electron microscopy (TEM) images were taken using a Philips CM-20 SuperTwin microscope. The samples were dispersed in methanol, and a drop of prepared suspension was put on a copper grid. Next, the samples were dried and purified in a plasma cleaner. Studies were performed in a conventional TEM procedure with a parallel beam electron energy of 1160 kV. The size of the obtained powders was determined manually using ImageJ software by measuring the longest linear size (Feret diameter) of each particle. The K₂Ge₄O₉ material was investigated by Raman spectroscopy in the pressure range from ≈0 to 10 GPa, in a backscattering geometry using a Renishaw inVia confocal micro-Raman system with a power-adjustable 100 mW 532 nm laser. An optical system with an Olympus x20 SLMPlan N long working distance objective was used to focus the laser beam. Vibrational analysis for the sample compressed in a pressure transmitting medium (methanol : ethanol : water – 16 : 3 : 1) was conducted in a diamond anvil cell (DAC) equipped with the ultra-low fluorescence diamond anvils (IIas type). Excitation and emission spectra were measured on an FLS1000 spectrometer from Edinburgh Instruments supplied with a 450 W xenon lamp as an excitation source, and the R928P side window photomultiplier tube from Hamamatsu as a detector. The emission spectra and luminescence decay profiles as a function of pressure and temperature were measured using the same system with a 445 nm laser diode as the excitation source. A THMS 600 heating–cooling stage from Linkam was utilized to control the temperature with a precision of 0.1 K. Pressure-dependent luminescence studies were conducted in a gas (nitrogen) membrane-driven diamond anvil cell Diacell μScopeDAC-RT(G) from Almax easyLab with a 0.4 mm diamond culets. The sample and pressure indicator Al₂O₃:Cr³⁺ were placed in a ≈140 μm hole drilled in a 250 μm thick stainless-steel gasket. The pressure transmission medium was methanol : ethanol : water – (16 : 3 : 1).

A Druck PACE 5000 was used to control the applied pressure. Accurate determination of the pressure in the DAC was possible by controlling the spectral shift of the R1 ruby (Al₂O₃:Cr³⁺) line.

Results and discussion

The K₂Ge₄O₉ crystallizes in a trigonal structure with P3c1 space group (No. 165) with lattice parameters a = 11.84, b = 11.84 and c = 9.8 (Å).^33–36 The structure of the K₂Ge₄O₉, similarly like in the case of the family of tetragermanates of alkali ions, consists of characteristic Ge₃O₉ rings that form layers in the ab planes (Fig. 1a). As it can be seen, there are four nonequivalent Ge sites in the structure, i.e. two tetrahedral (Ge(2) and Ge(3)) and two octahedral (Ge(1) and Ge(4)). The Ge₃O₉ rings consist of two Ge(3)O₄ and one Ge(2)O₄ thus created planes are connected by each other through two types of Ge(1)O₆ and Ge(2)O₆ octahedra by sharing O²⁻ ions. Due to the size and the preferred coordination only octahedral sites can be occupied by the Mn⁴⁺ ions. Therefore only Ge(1) and Ge(4) can be considered as potential sites replaced by the dopant ions in the K₂Ge₄O₉ structure. As calculated by Redhammer et al. the Ge(4) is a bit smaller than Ge(1), and in contrast to the Ge(4), the Ge(1) site exhibits inversion symmetry.^35,37 Therefore it is expected that the emission spectra of K₂Ge₄O₉:Mn⁴⁺ will be dominated by the signal of the Mn⁴⁺ from the low-symmetry Ge(4) site. Based on the XRD data it was found that in the case of the stoichiometric amounts of the reagents due to the lability of K⁺ ions, additional reflections were found in the diffractograms revealing the presence of the GeO₂ phase (Fig. 1b). The intensity of those reflections decreases gradually with the enlarging concentration of the excessing K₂CO₃ and finally for the 20% molar excess the pure K₂Ge₄O₉:Mn⁴⁺ powders were obtained. Therefore this reactant molar ratio was used for further synthesis. The analysis of the XRD patterns obtained for different concentrations of Mn⁴⁺ ions indicates the lack of additional X-ray reflexes confirming the phase purity of the obtained materials (Fig. 1c). However, in the case of the sample with 5% of Mn⁴⁺ ions, some broad band at low 2-theta angles was observed in the XRD pattern, most probably indicating the partial amorphization of the K₂G₄O₉ structure. Therefore the analysis discussed in this paper was limited to the 0.1–2% Mn⁴⁺ dopant concentration. Based on the Rietveld refinement of the obtained XRD patterns, the unit cell volume was determined as a function of the Mn⁴⁺ concentration. The contraction of the cell volume, resulting from the difference in the ionic radii between Ge⁴⁺ and Mn⁴⁺ ions, confirms the successful replacement of host cations by the dopant ions (Fig. 1d, see Table S1 for Rietveld refinement parameters, ESI†). The SEM images reveal that the obtained powders consist of aggregated particles of average size around 2.5 μm in diameter (Fig. 1e), whereas the EDS spectroscopy validates the uniform distribution of all constituent ions and dopant ions (Fig. 1f–i).

Fig. 1 Visualization of the K₂Ge₄O₉ structure (a), XRD patterns of the K₂Ge₄O₉:Mn⁴⁺ for different molar excess of K₂CO₃ (b), comparison of XRD patterns of the K₂Ge₄O₉:Mn⁴⁺ for different concentration of Mn⁴⁺ ions (c), the volume of the unit cell of the K₂Ge₄O₉:Mn⁴⁺ as a function of Mn⁴⁺ concentration determined from Rietveld refinement (d), representative SEM image of the K₂Ge₄O₉:0.1%Mn⁴⁺ (e), and corresponding EDS maps of the elemental distribution of K (f), Ge (g), O (h) and Mn (i).

The structural stability of the investigated K₂Ge₄O₉ material was analyzed by measuring the Raman scattering spectra with pressure. The recorded spectrum for the investigated material at 0.46 GPa (initial pressure value obtained in a DAC) has seven bands initially located around ≈240 and 400-internal vibrations, 515, 535 and 565 – the symmetric bending vibration of Ge–O–Ge, 820 and 900 cm⁻¹ – the symmetric stretching vibration of Ge–O–Ge (Fig. 2a and b). Under the compression process (increasing pressure in the system) the energies of all phonon modes increase, and the Raman modes centroids shift toward higher wavenumbers in the vibrational spectra, as shown in Fig. 2b. The calculated Raman mode spectral shifts with pressure are gradual and linear, which confirms the stability of the crystal structure of the investigated materials. The most intense Raman mode at ≈515 cm⁻¹ shows the highest value of linear shift around 5.70 cm⁻¹ GPa⁻¹. The calculated shift rates (cm⁻¹ GPa⁻¹) for the observed Raman modes are gathered in Table S1 (ESI†). The observed effects are the results of the decrease of the interatomic distances in the structures under compression, i.e. bond shortening. Additionally, some changes in the Raman modes intensity ratio (between ≈3.5–5.5 GPa), as well as the vanishing and emerging of the corresponding two peaks (at around ≈560 cm⁻¹) were observed. This effect may indicate a gradual pressure-induced phase transition of the K₂Ge₄O₉ material, which is reversible, as confirmed by decompression data. The high signal-to-noise ratio for all measured Raman spectra (even under extreme values of high pressure) is related to the very good crystallinity of the K₂Ge₄O₉ material. It may also indicate that crystal defects and strains, which are typically observed during high-pressure Raman measurements, do not play a significant role during the compression process in this case, for this particular material. Moreover, in the decompression cycle, the reverse tendencies of spectral shift and the shape of Raman modes were observed, namely, they fully come back to the initial state upon pressure release (see Fig. 2b and Fig. S1, ESI†). In other words, no plastic deformations during the compression–decompression cycle were observed. The same position of initial Raman modes before compression and after the decompression cycle is evidence of reversibility of the whole compression process, including structural changes and phase transition in the K₂Ge₄O₉ material. This factor is an important aspect in designing new optical manometers, so the examined K₂Ge₄O₉ material might be considered as a potential contactless pressure sensor.

Fig. 2 Normalized Raman spectra for the K₂Ge₄O₉ material in the compression process (a). Calculated peak centroids of phonon modes under compression (b); spheres present peak centroid values in compression, empty circles represent peak centroids in the decompression process; the lines are the linear function fitted for determination of the pressure-induced shift rates of the Raman modes.

Mn⁴⁺ ions are well-known in the literature and are often used as luminescent dopants providing the possibility of generation of deep-red emission. The luminescence properties of these ions are related to the spin-forbidden ²E → ⁴A₂ electronic transition. The ²E level, unlike the ⁴T₂ and ⁴T₁ levels due to the absence of a shift of its parabola in the wavevector domain with respect to the ⁴A₂ ground state, is located directly above the bottom of the ⁴A₂ level. Therefore, the radiative transition between ²E and ⁴A₂ is represented in the emission spectra as a spectrally narrow band, often accompanied by lines associated with phonon progression (Fig. 3a). In the case of the K₂Ge₄O₉ compound, the ²E → ⁴A₂ emission band is located at around 670 nm (R line at 650 nm), and its shape is independent of the dopant ion concentration (Fig. 3b). The excitation spectrum of the K₂Ge₄O₉:Mn⁴⁺ consists of a group of spectrally broad bands located in the range of 260–563 nm (38 532–17 746 cm⁻¹). The deconvolution of the excitation spectrum allows to indicate the maxima of these bands: ⁴A₂ → ⁴T₂ (21 920 cm⁻¹), ⁴A₂ → ²T₂ (24 795 cm⁻¹); ⁴A₂ → ⁴T₁ (29 320 cm⁻¹) and Mn⁴⁺ → O²⁻ charge transfer band (33 527 cm⁻¹) (Fig. 3c). An increase in temperature results in a gradual thermalization of higher lying vibrational states of the ²E level, until the delivered thermal energy is sufficient to exceed the intersection point between the ²E and ⁴T₂ states, resulting in nonradiative depopulation of the ²E level. The relatively large value of the energy of this crossover point (usually >1000 cm⁻¹)³⁸ results in the high thermal stability of Mn⁴⁺ luminescence. The energies of the ⁴T_J states are strongly dependent on the crystal field strength affecting the Mn⁴⁺ ions. Therefore, the compression of the host material by applied external pressure causes the increase of energy of ⁴T_J inducing all the associated spectroscopic consequences discussed later in this paper. Although the shape of the emission spectrum changes negligibly with varying concentrations of dopant ions, the intensity of the luminescence decreases significantly (Fig. 3d). The maximum luminescence intensity is observed for the sample with 0.1% of Mn⁴⁺ (exceeding twice the intensity of luminescence obtained for the one with 0.2% of Mn⁴⁺). Therefore, this dopant ion concentration was used for high-pressure research. The analysis of the luminescence kinetics revealed that as the dopant ion concentration increases, the luminescence decay profiles from the ²E level deviate from the exponential character (luminescence decay curve can be found in Fig. S3, ESI†). Therefore, in order to provide a comparative analysis of the effect of dopant ion concentration on the lifetime of the ²E level, the average lifetimes were determined using (eqn (1)) based on parameters obtained by fitting the measured luminescence decay profiles to the double exponential function (eqn (2)).

where A₁, A₂, τ₁ and τ₂ are the fitting parameters. The analysis showed that for the concentrations of 0.1–0.2% of Mn⁴⁺ the τ_avr = 0.98 ms, and shortens linearly with increasing concentration of Mn⁴⁺ ions, reaching 0.71 ms at 2% of Mn⁴⁺.

Fig. 3 The simplified configurational coordination diagram of Mn⁴⁺ ions at ambient and high-pressure conditions (a); the comparison of the normalized emission spectra of the K₂Ge₄O₉:Mn⁴⁺ measured at 83 K upon λ_exc = 445 nm for different Mn⁴⁺ concentration (b); the deconvolution of the excitation spectrum of the K₂Ge₄O₉:0.1%Mn⁴⁺ sample measured at 83 K for λ_em = 651 nm (c); comparison of the integrated emission intensity of the K₂Ge₄O₉:Mn⁴⁺ samples at room-temperature (d) and τ_avr (e) as a function of the Mn⁴⁺ concentration.

The Mn⁴⁺ ions with the 3d³ electronic configuration similarly to Cr³⁺ ions are extremely sensitive to changes in the strength of the crystal field. Therefore, the applied pressure (leading to the contraction of the unit cell and consequent shortening of the Mn⁴⁺–O²⁻ distances) increases the strength of the crystal field acting on Mn⁴⁺ ions. A careful analysis of the Tanabe–Sugano diagram for ions of 3d³ configuration indicates that such a change in the strength of the crystal field leads to an increase in the energy of ⁴T_J levels. In contrast, the energy of the ²E state reveals the independence of the changes in the strength of the crystal field. However, shortening the Mn⁴⁺–O²⁻ distance enhances the covalency of this bond, which, in turn, through the nephelauxetic effect, slightly decreases the energy of the ²E level, resulting in a red-shift of the ²E → ⁴A₂ band with pressure (Fig. 4a, see also Fig. S4, ESI†). This is an effect commonly used for pressure sensing in the case of the well-known ruby (Al₂O₃:Cr³⁺) sensor. In the case of the K₂Ge₄O₉:Mn⁴⁺, the applied pressure leads to a shift of the R-line from 15 332 cm⁻¹ at ambient pressure to 15 238 cm⁻¹ at 7.6 GPa. In addition, as can be seen in Fig. 4a, an increase in pressure results in a broadening of emission lines and a reduction in their emission intensity. Although the data presented in Fig. 4a are normalized, an increase in noise-to-signal ratio as pressure increases can be recognized. A decrease in emission intensity with increasing pressure is very common in high-pressure spectroscopy, and in this case it may result from a growing number of strains and defect states that act as quenching centers, as well as from improved cross-relaxation processes in a compressed structure. A detailed analysis of the excitation spectra of Mn⁴⁺ ions shows, as expected, an increase in the energy of the bands associated with the ⁴A₂ → ⁴T_i transitions (Fig. 4b). The observed change in band energies amounted to 1987 cm⁻¹ (from 21 964 cm⁻¹ at ambient pressure to 23 951 cm⁻¹ at 7.6 GPa) for the ⁴A₂ → ⁴T₂ band and 1326 cm⁻¹ (from 33 654 cm⁻¹ at ambient pressure to 34 980 cm⁻¹ at 7.6 GPa) for the ⁴A₂ → ⁴T₁ band. The data obtained allowed the determination of the crystal field Dq and Racah B and C parameters as follows:^39–41

Fig. 4 The normalized room-temperature emission spectra (λ_exc = 445 nm) of the K₂Ge₄O₉:0.1%Mn⁴⁺ material (a) and excitation spectra (λ_em = 651 nm) (b) measured as a function of applied pressure, and the corresponding B (c), Dq/B (d) and β₁ (e) parameters calculated based on the excitation spectra.

As observed, the B parameter gradually decreases with pressure, which is a typical effect observed for materials doped with Mn⁴⁺ ions (Fig. 4c, for pressure dependence of C parameter see Fig. S5, ESI†).^38,42,43 Its value at ambient pressure equals 757 cm⁻¹, which corresponds to relatively high values of crystal field strength. On the other hand, the Dq/B parameter increases significantly with pressure, i.e. from 2.85 at ambient pressure to about 3.47 at 7.6 GPa (Fig. 4d). The nephelauxetic effect as proposed by Brik et al. can be analyzed through parameter β₁ defined as follows:^44,45

where B₀ and C₀ are the values for free ions equal to 1160 and 4303 cm⁻¹, respectively.⁴⁵ An increase in the covalency of the Mn⁴⁺–O²⁻ bond is confirmed by a monotonic decrease in the B₁ parameter with increasing pressure (Fig. 4e). Notably, the observed reduction in the B₁ parameter is greater (1.9% change) than for the recently reported Sr₄Al₁₄O₂₅:Mn⁴⁺ material (0.25% change in the analogous pressure range).¹⁸

The monotonic change in the energy maximum of the ²E → ⁴A₂ band or the corresponding change in the spectral position of this band as a function of the applied pressure makes it possible to develop a spectral-shift based luminescence manometer. It can be clearly seen that in the analyzed pressure range, a change in the energy of the ²E level of about 80 cm⁻¹ is observed, which corresponds to a change in the spectral position of the band of about 3 nm (Fig. 5a). Quantitative analysis of the spectral band shift is usually done by determining the absolute sensitivity (S_A,p) as follows:

Fig. 5 Change in the energy and wavelength of the R line of the ²E → ⁴A₂ emission band maxima as a function of pressure (a) and corresponding S_A (b); emission spectra of K₂Ge₄O₉:0.1%Mn⁴⁺ measured as a function of temperature at ambient pressure (λ_exc = 445 nm) (c) thermal dependence of the change in the energy and wavelength of the R line of the ²E → ⁴A₂ emission band maxima (d) and corresponding S_A (e).

In the case of the K₂Ge₄O₉:0.1%Mn⁴⁺ phosphor, the value of S_A,p increases with pressure up to about 5 GPa, at which point S_A,p reaches a maximum (S_A,p(cm⁻¹) = 12 cm⁻¹/GPa; S_A,p(nm) = 0.59 nm GPa⁻¹). Beyond this pressure, a decrease in S_A,p is observed (Fig. 5b). The sensitivity values obtained for the K₂Ge₄O₉:0.1%Mn⁴⁺ material fall within the typical range of spectral shifts observed for transition metal ions with a 3d³ electronic configuration when emission occurs from the ²E level. Importantly, the achieved S_A,p value exceeds the sensitivity of the ruby sensor – Al₂O₃:Cr³⁺ (S_A,p = 0.365 nm GPa⁻¹).⁴⁶ For real-world applications, a change in pressure may be accompanied by a change in the temperature of the analyzed object. Therefore, low sensitivity to temperature changes of the luminescent manometer is essential to maintain high reliability of pressure value determination. For this purpose, emission spectra of K₂Ge₄O₉:0.1%Mn⁴⁺ were measured as a function of temperature (Fig. 5c) the thermal sensitivity of the manometer is analyzed, determined in a manner analogous to S_A,p:

An increase in temperature causes a successive decrease in the intensity of the ²E → ⁴A₂ emission associated with the thermalization of the ⁴T₂ level, followed by nonradiative transitions to the ⁴A₂ ground state. Consequently, the luminescence intensity at 423 K is low. Therefore, the analysis was limited to the 83–423 K range. In this temperature range, a gradual shift in the energy of this energy level of about 100 cm⁻¹ (4 nm) was observed (Fig. 5d), which corresponds to the S_A,T of 0.45 cm⁻¹ K⁻¹ (0.02 nm K⁻¹), as shown in Fig. 5e. This is almost three times higher than that observed for Al₂O₃:Cr³⁺ (S_A,T = 0.007 nm K⁻¹),^47,48 which reduces the reliability of the pressure reading using the spectral shift of the emission band of Mn⁴⁺ ions in the K₂Ge₄O₉:0.1%Mn⁴⁺.

However, it is noteworthy that a pronounced thermal shift in the spectral position of the R-line (which is very narrow) associated with Mn⁴⁺ ions is exceptionally rare, rendering it an exceedingly precise temperature indicator. While literature commonly discusses the spectral shift of a band as a parameter of limited utility for temperature imaging, given the necessity for point-by-point measurements across emission spectra, a single-point reading of such a spectrally narrow line, exhibiting high sensitivity to temperature variations, proves highly advantageous for other (than imaging) temperature monitoring purposes. This capability facilitates accurate temperature determination without the need for measurements across a wide spectral range. The dual sensitivity of the same parameter to both temperature and pressure changes may be perceived as a potential limitation in sensor applications. However, it is pertinent to note that within the pressure range typically employed in most industrial applications (up to ∼10⁻¹ GPa), the pressure-induced shift of the R-line is merely 0.4 cm⁻¹. In contrast, within the analyzed temperature range, a thermal shift exceeding 100 cm⁻¹ is observed. The pressure sensitivity of the spectral position of the Mn⁴⁺ emission band in K₂Ge₄O₉:0.1%Mn⁴⁺ is significantly lower compared to other already reported Mn⁴⁺ luminescence manometers like Gd₂ZnTiO₆ of 1.11 nm GPa⁻¹ sensitivity in the spectral shift readout mode.⁴⁹ Consequently, K₂Ge₄O₉:0.1%Mn⁴⁺ can be effectively employed as a bi-function sensor capable of concurrently measuring temperature and pressure under extreme conditions, showcasing its versatility and utility across diverse applications.

Although spectral shift is the most commonly used spectroscopic parameter for luminescent pressure sensors, its use is difficult or impossible in certain applications. Imaging pressure changes over a larger area (via 2D/3D mapping) using spectral-shift based manometers would be very time consuming, as requiring point-by-point measurement of emission spectra. Therefore, an alternative strategy reported recently in the literature is the ratiometric approach, in which the ratio of luminescence intensities in two spectral regions (LIR) is a manometric parameter.

The analysis of the K₂Ge₄O₉:0.1%Mn⁴⁺ emission spectra as a function of pressure indicates that the R-line not only undergoes a spectral shift, but also decreases in intensity with respect to the lines associated with phonon progression. Therefore, the LIR is defined as follows:

shows high variability as a function of pressure (in the wavenumber domain the LIR can be calculated in the following regions 15 243–15 408 cm⁻¹/14 858–14 706 cm⁻¹). As presented in Fig. 6a the maximum value was reached at about 3 GPa, and then the LIR parameter decreased monotonically with further pressure increase. The value of the pressure at which the maximum sensitivity was reached correlates well with the pressure value above which some changes in the vibronic spectra were found. Therefore probably the changes in the S_R are associated with the structural changes of the host material. However, the measurement during the compression–decompression cycles confirmed that observed changes are reversible. Therefore, the relative sensitivity (S_R,p) is defined as follows:reaches negative values in the pressure range below 3 GPa, and increases with pressure up to 5.9 GPa, at which point S_R,p = 21.7%/GPa was obtained (Fig. 6b). Of course, the negative S_R values are only due to the conventions of LIR calculation. However, changing the monotonicity of the gauge operating parameter strongly limits the useful range of pressure values over which it can be used, because otherwise one LIR value would correspond to two different pressure values making the gauge useless. Therefore, it should be clearly underlined here that for K₂Ge₄O₉:0.1%Mn⁴⁺ the LIR can be used as a sensing parameter for pressure above 3 GPa. Importantly the thermal relative sensitivity is defined by analogy to pressure sensitivity:reaches values of about S_R,T = 1.3%/GPa around 140 K, and above 200 K it drops almost to zero (Fig. 6d), which confirms the higher thermal invariability of LIR sensing mode (Fig. 6c) compared to a manometer based on the spectral shift. A quantitative manifestation of the thermal invariability of the manometer parameter to pressure changes is provided by the determination of the TIMF parameter (thermal invariability manometric factor) defined as follows:²⁹

Fig. 6 Pressure –(a) and thermal –(b) dependences of the LIR and corresponding S_R- (c) and (d), respectively, in K₂Ge₄O₉:0.1%Mn⁴⁺.

The TIMF parameter determines how much change in absolute temperature (in K) is required to modify the value of the manometric parameter corresponding to the change in pressure by 1 GPa. Ideally, for a luminescent pressure gauge that is insensitive to temperature changes, the TIMF should approach infinity. However, technically, it can be assumed that the manometer reveals adequate insensitivity to temperature changes when TIMF > 100²⁹. Since thermal sensitivity depends on temperature, it is recommended to determine the TIMF in the operating temperature of the manometer, which in this case is around 300 K. In the case of the K₂Ge₄O₉:0.1%Mn⁴⁺ sensor operating in the ratiometric mode the TIMF = 5425 K/GPa (S_R,T@300 K = 0.004%/K), which indicates high reliability of K₂Ge₄O₉:0.1%Mn⁴⁺ as a ratiometric luminescent manometer, allowing its use at extreme conditions of high pressure and temperature.

Due to the fact that the ²E level is located below the ⁴T₂ state and that the usual kinetics from the ²E level is much longer (∼ms) than that of the ⁴T₂ level (∼tens of μs), the luminescence decay profile of Mn⁴⁺ ions is dominated by the signal associated with the depopulation of the ²E level. However, the ²E and ⁴T₂ levels are coupled to each other by the spin-orbital interaction. This interaction shortens the lifetime of the ²E level and increases the probability of its radiative depopulation. Therefore, when the applied pressure elevates the energy of the ⁴T₂ level, the spin-orbital coupling is weakened and thus the luminescence decay time is prolonged. This effect was used in the case of the K₂Ge₄O₉:0.1%Mn⁴⁺ material to develop a lifetime-based luminescence manometer. As can be clearly seen, an increase in isostatic pressure significantly affects the luminescence decay profile of Mn⁴⁺ ions by its elongation and more exponential course (Fig. 7a). The determined values of the average lifetime (τ_avr) increase monotonically from 0.97 ms at ambient pressure to 1.55 ms at 6 GPa (Fig. 7b). Above this value, a saturation of decay time values is observed. Quantitative analysis of the change in the lifetime of the ²E level as a function of pressure done by determining relative sensitivity showed that the S_R,p value increases from 4%/GPa at ambient conditions up to 12%/GPa at 3.3 GPa, and then decreases to 0.8%/GPa at 7.2 GPa (Fig. 7c). Notably, the effect observed for the Mn⁴⁺ emission kinetics is in contrast to manometers based on the luminescence kinetics of lanthanide-doped materials, where both the applied pressure and temperature cause the analogous direction of change (shortening or prolongation tendency) in excited state lifetime. To verify this hypothesis for the K₂Ge₄O₉:0.1%Mn⁴⁺ material, an analysis of the luminescence decay profile as a function of temperature in the range of 83–423 K was performed (Fig. 7d). Elevation of temperature results in a decrease in the τ_avr with a low quenching rate, up to around 350 K, above which point a sharp reduction in τ_avr value, down to 0.01 ms at 423 K, is recorded (Fig. 7e). This rapid effect observed above 350 K is caused by the depopulation of the ²E level by thermalization of the ⁴T₂ state. Therefore, the relative thermal sensitivity below ≈350 K is lower than 1%/K, and a further temperature increase enhances sensitivity value up to S_R,T = 4%/K at around 423 K (Fig. 6f).

Fig. 7 The luminescence decay profile of the ²E state in K₂Ge₄O₉:0.1%Mn⁴⁺ (λ_exc = 445 nm, λ_em = 651 nm) measured at room temperature as a function of applied pressure (a) and corresponding τ_avr (b) and S_R (c); the influence of the temperature on the luminescence decay profile of the ²E state in K₂Ge₄O₉:0.1%Mn⁴⁺ (λ_exc = 445 nm, λ_em = 651 nm) at ambient pressure (d) and corresponding τ_avr (e) and S_R (f).

Although the determined maximum sensitivity of the lifetime-based luminescence manometer K₂Ge₄O₉:0.1%Mn⁴⁺, i.e. S_R = 12%/GPa, is lower than the maximum value for the Sr₄Al₁₄O₂₅:Mn⁴⁺ (S_R = 35%/GPa at 0.8 GPa),¹⁸ it should be noted that at higher pressure values (above 1.5 GPa) the relative sensitivity for Sr₄Al₁₄O₂₅:Mn⁴⁺ drops dramatically, and at 3.3 GPa is already much below 1%/GPa. In this pressure range of around 3 GPa, definitely more pronounced manometric performance is shown for SrGdAlO₄:Mn⁴⁺ with a relative sensitivity of S_R = 7.85%/GPa.¹⁹ However, the value of S_R = 12%/GPa undoubtedly confirms the predominance of K₂Ge₄O₉:0.1%Mn⁴⁺ in this pressure range around 3 GPa confirming its significant application advantages. The performed comparison indicates that the sensitivity of the luminescent manometer based on the emission kinetics of Mn⁴⁺ ions is strongly affected by the composition host material. Therefore, the selection of a suitable phosphor for luminescence based pressure sensing should be dictated by the requirements imposed by the type of application.

Conclusion

In this work, the influence of applied pressure on the spectroscopic properties of K₂Ge₄O₉:Mn⁴⁺ crystals was investigated in order to develop a highly sensitive, multi-parameter luminescent manometer based on sensing in three different modes, namely, (I) the kinetics of the ²E state of Mn⁴⁺; (II) the emission line shift; and (III) LIR. The quenching of the luminescence intensity and shortening of the lifetime of the ²E excited state with an increase of the Mn⁴⁺ concentration motivated the selection of 0.1% of Mn⁴⁺ as an optimal concentration of dopant ions for further experiments. The performed high-pressure studies of the spectroscopic properties of the K₂Ge₄O₉:0.1% Mn⁴⁺ material indicate that due to the nephelauxetic effect associated with the change in the covalency of the Mn⁴⁺–O²⁻ bond, the energy of the ²E → ⁴A₂ emission band decreases monotonically with a shift rate of 0.59 nm GPa⁻¹ (12 cm⁻¹ GPa⁻¹). However, the determined thermal dependence of the spectral position of the R line was relatively large, i.e. 0.02 nm K⁻¹ (0.45 cm⁻¹ K⁻¹). Above 3 GPa the intensity of the R line started to decrease with respect to the phonon progression lines, enabling the development of the ratiometric luminescence manometer operating in the 3–7.6 GPa p-range, with the maximal relative sensitivity of S_R = 21.7%/GPa at 5.9 GPa, and a TIMF factor of 5425 K GPa⁻¹ at 300 K. Another effect associated with the applied pressure was the reduction of the spin–orbital coupling between ²E and ⁴T₂ states, related to the enhancement of the energy of the ⁴T₂ state under structure compression. As a consequence, the luminescence decay profile of the ²E state of Mn⁴⁺ ions elongates and becomes more exponential with pressure. The monotonic prolongation of τ_avr in the 0.3–6 GPa p-range enables the development of a lifetime-based manometer, characterized by S_Rmax = 12%/GPa at 3 GPa. This is, to the best of our knowledge, the most sensitive lifetime-based luminescent manometer operating in the pressure range above 2 GPa reported so far. Additionally, the strong thermal susceptibility of the R-line of Mn⁴⁺ ions in K₂Ge₄O₉ (0.02 nm K⁻¹), together with its negligible pressure-induced shift in the low pressure regime, enables the application of this phosphor as a bi-functional pressure–temperature sensor, in both scientific and industrial applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Center (NCN) Poland under project no. DEC-UMO-2020/37/B/ST5/00164. W. M. P. acknowledges the support from the Foundation for Polish Science (FNP) under the START programme.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3tc04812h

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