Synthesis, crystal structure, optical and thermal properties of lanthanide hydrogen-polyphosphates Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho) †

Lanthanide hydrogen-polyphosphates Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho) were synthesised as colourless (Ln = Tb, Dy) and light pink (Ln = Ho) crystalline powders by reaction of Tb 4 O 7 /Dy 2 O 3 /Ho 2 O 3 with H 3 PO 3 at 380 °C. All compounds crystallise isotypically ( P 2 1 / c (no. 14), Z = 4, a Tb = 1368.24(4) pm, b Tb = 710.42(2) pm, c Tb = 965.79(3) pm, β Tb = 101.200(1)°, 3112 data, 160 parameters, w R 2 = 0.062, a Ho = 1363.34(5) pm, b Ho = 709.24(3) pm, c Ho = 959.07(4) pm, β Ho = 101.055(1)°, 1607 data, 158 parameters, w R 2 = 0.058). The crystal structure comprises two di ﬀ erent in ﬁ nite helical chains of corner-sharing phosphate tetrahedra. In-between these chains the lanthanide ions are located, coordinated by seven oxygen atoms belonging to four di ﬀ erent polyphosphate chains. Vibrational, UV/Vis and ﬂ uorescence spectra of Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho) as well as Dy[H(PO 3 ) 4 ]:Ln (Ln = Ce, Eu) and the magnetic and thermal behaviour of Tb[H(PO 3 ) 4 ] are reported.

The protonated chain (A) reveals a larger diameter compared to the non-protonated polyphosphate chain (B) (Fig. 2) due to the widened P-O-P angles (136°in A vs. 130-133°in B).
The adjacent A and B chains in the a and c directions, respectively, exhibit a phase shift of 180°towards each other (Fig. 3).
In the phosphate tetrahedra P-O bond lengths range between 146 and 161 pm.The P-O H bond is 153 pm long.P-O br distances range between 156 and 161 pm, whereas P-O term distances range between 146 and 150 pm.9][20][21][22][23] In general, P-O distances increase from P-O term to P-O H and up to P-O br due to the decreasing effective negative charge on the involved oxygen atom which leads to weaker electrostatic interactions between P and O. Angles of the P OH -O br -P O and P O -O br -P O bridges range between 135 and 137°and between 129          10.
An appropriate method for the calculation of the deviation of tetrahedra from ideal symmetry was introduced by Balić-Žunić and Makovicky. 24,25We adopted and explained the application of this method with the example of α-BaHPO 4 ; 26 in the course of our recent results we assigned deviations of less than 1% to regular tetrahedra.The four crystallographically     4).The singlecrystal structure reveals a single site of Ln 3+ , which is coordinated capped trigonal prismatic by seven terminal oxygen atoms of the polyphosphate chains (Fig. 5 and 6).The Ln-O bond lengths range between 225 and 245 pm (Table 10).These values are in the same range as the Er-O bond lengths in Er[H(PO 3 ) 4 ] determined by Palkina et al. 17 In Ln[H(PO 3 ) 4 ] (Ln = Tb, Ho) a single hydrogen bond was found which can be considered as moderate according to       6).In contrast to moderate hydrogen bonds with a hydrogen to acceptor distance between 150 and 220 pm and bond angles ∢(DHA) larger than 130°, strong hydrogen bonds exhibit a distance of 120-150 pm and an angle of 170-180°.
The crystal structure of Dy[H(PO 3 ) 4 ] was refined via the Rietveld method (Fig. 8 and Table 11) based on the structure model of Tb[H(PO 3 ) 4 ].

Electrostatic calculations
2][33] The presence of one proton could also be proven.A structure model is electrostatically consistent if the sum of MAPLE values of chemically similar compounds deviates from the MAPLE value of the compound of interest by less than approximately 1%.Thus the structure models of Tb[H(PO 3 ) 4 ] and Ho[H(PO 3 ) 4 ] show electrostatic consistency (Table 1).
The bands between 1350 and 1200 cm −1 can be assigned to the asymmetric stretching vibrations of PO 2 , while vibrations between 1200 and 990 cm −1 can be assigned to the stretching vibrations of PO term .7][38] The four symmetric stretching modes of the PO br vibrations are detected between 780 and 650 cm −1 peaking at 681, 700, 741 and 770 cm −1 , which had been empirically correlated with the periodicity of polyphosphate chains in catena-polyphosphates. 11,36,39This also holds for Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho), due to the periodicity P = 4 of its poly-hydrogenphosphate chains.7][38] Surpris-   ingly, no vibrations of the hydroxyl groups were detected, which leads to the assumption that presumably the ratio of O-H/P-O vibrations of 1 : 16 is too small to be detected via ATR.

UV/Vis spectroscopy
The UV/Vis reflection spectra of Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho) (Fig. 10-12) reveal the typical absorption bands of the f-f transitions of Tb 3+ , Dy 3+ and Ho 3+ ions, according to the wellknown energy level schemes. 40,41All transitions start from the respective ground states 7  shows the alexandrite effect. 43,44Due to an absorption gap in the yellow region, it exhibits a yellow body colour in ambient day light.In artificial light, e.g.fluorescent lamps, exhibiting distinct emissions in the blue (∼450 nm), green (∼540 nm) and red (∼610 nm) range, it reveals a pink body colour, due to the reflection of the wavelengths between 550 and 620 nm. 4

Fluorescence spectroscopy
][42] Between 230 and 267 nm a broader band in the excitation spectrum of Tb[H(PO 3 ) 4 ] reveals the parity allowed 4f 8 →     showing the highest intensity, 5 D 4 row 7 F 4 (583 nm) and 5 D 4 → 7 F 3 (619 nm) transitions. 42e excitation spectrum of Dy[H(PO 3 ) 4 ] exhibits a broad band around 290 nm, which reveals the 4f 9 → 4f 8 5d 1 transition.Under excitation at 349 nm Dy[H(PO 3 ) 4 ] exhibits sharp emission lines between 460 and 675 nm, which correspond to the f-f electronic transitions 4 F 9/2 → 6 H 15/2 (482 nm), 4 F 9/2 → 6 H 13/2 (573 nm) and 4 F 9/2 → 6 H 11/2 (671 nm). 41The most intense transition 4 F 9/2 → 6 H 13/2 is sensitive to the surrounding of Dy 3+ , and is thus called hypersensitive.The surrounding of Dy 3+ in Dy[H(PO 3 ) 4 ] is of relatively low symmetry (single capped trigonal prismatic), therefore the intensity of the hypersensitive transition is increased with respect to the 4 F 9/2 → 6 H 15/2 transition. 45,46ping of Dy[H(PO 3 ) 4 ] with increasing concentrations of Ce 3+ (5, 10, 30 mol%) was carried out to investigate the effect of Ce 3+ as a sensitizer.The presence of cerium and the corresponding ratios of Dy : Ce were investigated via EDX spectroscopy.The resulting ratios are 1 : 0.04 (expected: 1 : 0.05), 1 : 0.07 (1 : 0.10) and 1 : 0.26 (1 : 0.30), which are within the limits of accuracy of the measurements very close to the expected values.Due to an overlap in the wavelength range of the emission bands of Ce 3+ and the excitation spectrum of Dy 3+ energy transfer could be expected.Transitions in Ce 3+ are f-d transitions, which are allowed due to the selection rules, and thus lead to broad and intense bands in the fluorescence spectrum. 13The fluorescence spectra of the doped samples Dy[H(PO 3 ) 4 ]:Ce 3+ are shown in Fig. 15.The excitation spectra exhibit strongly increasing broad band intensities of Ce 3+ at 302 nm ( 2 F 5/2 → 5d).In the emission spectra, next to the typical broad band emissions of Ce 3+ peaking at 317 nm (5d → 2 F 5/2 ) and 336 nm (5d → 2 F 7/2 ) 47,48 two comparably weak emis-sion bands of Dy 3+ , corresponding to the f-f electronic transitions 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 , can be observed.Despite the low Dy 3+ emission intensities the hypersensitive transition 4 F 9/2 → 6 H 13/2 can still be observed with higher intensity compared to the 4 F 9/2 → 6 H 15/2 transition.Doubling the content of Ce 3+ from 5 to 10% the emission intensity of Dy 3+ also increases equally ( 4 F 9/2 → 6 H 15/2 ), whereas the hypersensitive transition 4 F 9/2 → 6 H 13/2 even triples.A further increase of the Ce 3+ content to 30% leads to a raised intensity of a factor of 2.5 for the 4 F 9/2 → 6 H 15/2 transition and a factor of 1.7 for the 4 F 9/2 → 6 H 13/2 transition.Thus the emission intensities of Dy 3+ increase slightly with the increasing Ce 3+ content but do not prove efficient Ce 3+ → Dy 3+ energy transfer.The intensity of the f-d transition in the excitation spectrum increases corresponding to the increasing Ce 3+ content.0][51] By reversing the Dy 3+ /Ce 3+ ratio and increasing the content of Ce 3+ to Ce 0.75 Dy 0.25 [H(PO 3 ) 4 ] efficient energy transfer might be reached, which represents a substitution process rather than a doping process.

Magnetic properties
Due to the presence of one proton and the thus postulated sum formula of Tb[H(PO 3 ) 4 ], the valence state of terbium was confirmed by the magnetic susceptibility measurement, which was recorded in a field of 1000 Oe over the temperature range of 1.8 K < T < 400 K (Fig. 17).In the whole temperature range the molar susceptibility obeys Curie's law (χ m = C/T ) very well with a Curie constant of C = 11.3792emu mol −1 K −1 .The Curie constant corresponds to an effective magnetic moment per Tb 3+ ion of μ eff = 9.54μ B , which is close to the theoretical value (μ eff = 9.72μ B ) and to experimental effective magnetic moments of Tb 3+ ions (9.7-9.8μB ). 55 8 Thermal analysis The thermal behaviour of Tb[H(PO 3 ) 4 ] was investigated between room temperature and 1450 °C (Fig. 18).The thermogravimetric curve revealed several undefined steps with a total mass loss of 15.2 wt% in the temperature range of 500-1450 °C.Assuming that besides 0.5 moles of H 2 O a further 0.5 moles of P 2 O 5 evaporate (theor.mass loss: 16.8 wt%) a final composition of TbP 3 O 9 may be assumed.After the thermal treatment the product was glazed and the expected composition could not be confirmed via X-ray powder diffraction.Temperature-dependent X-ray powder diffraction confirms that Tb[H(PO 3 ) 4 ] is stable up to 500 °C (Fig. 19).At 900 °C α-TbP 3 O 9 (grey) represents the main phase next to a small part of TbP 5 O 14 (marked with *). 56This leads to the conclusion that the assumption of losing half a mole of each, H 2 O and P 2 O 5 , in the thermal analysis is correct.Tb[H(PO 3 ) 4 ] exhibits infinite chains of [H(PO 3 ) 4 ] 3− and [P 3 O 9 ] 3− anions.Hence it is not surprising that thermal decomposition leads to the elimination of the hydroxyl groups by evaporation of water.Astonishingly, under nitrogen flow this process does not occur below 500 °C.
The thermal decomposition of Ln[H(PO 3 ) 4 ] (Ln = Y, Gd-Er) was determined between 350 and 600 °C by Selevich et al. 57 According to the authors all investigations were carried out in atmospheric humidity.Thus thermal analysis was carried out in air and naturally proceeds in a shorter and lower temperature range (350-600 °C) rather than under an inert atmosphere (500-1440 °C) as performed in this work.Since an increasing stability is to be expected with an increasing connectivity, the thermal stability of Tb[H(PO 3 ) 4 ], exhibiting infinite polyphosphate chains, is estimated to be higher than for noncondensed cyclotetraphosphates, e.g.Ba 2 (P 4 O 1 2)•3.5H 2 O, 27 which decomposes around 380 °C.In contrast, thermal decomposition of the ultraphosphate YP 5 O 1 4, which reveals a higher degree of condensation, does not occur until 760 °C. 58

Conclusions
In this contribution we demonstrated the crystal structures of Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho), which were solved and refined based on single-crystal X-ray diffraction (Tb and Ho) and refined by a Rietveld refinement (Dy), respectively.Via MAPLE calculations the presence of a single proton could be proved on the one hand, and on the other hand the electrostatic con-sistency of the structure model of Ln[H(PO 3 ) 4 ] (Ln = Tb, Ho) was confirmed.All the observed bands in the IR spectrum are in agreement with our structure model.The optical properties reveal the typical UV/Vis reflection and fluorescence spectra of Tb 3+ , Dy 3+ and Ho 3+ ions exhibiting their characteristic absorption and emission bands.Doping of Dy[H(PO 3 ) 4 ] with increasing amounts of Ce 3+ revealed only slightly more intense emission bands of Dy 3+ in the blue and yellow regions.Doping of Dy[H(PO 3 ) 4 ] with Eu 3+ leads to a partial occupancy of Dy 3+ sites, which is in accordance with the relatively high intensity of the hypersensitive 5 D 0 → 7 F 2 transition of Eu 3+ in a noncentrosymmetric surrounding.The magnetic measurements reveal no interaction between the Tb 3+ centres and confirmed the oxidation state of Tb 3+   Ho[H(PO 3 ) 4 ] was synthesised analogously to Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy) but could be obtained as phase pure yellow/pink powder without addition of H 2 O 2 to the reaction mixture.

X-Ray powder diffraction
The crystalline sample was finely ground, enclosed in a Hilgenberg glass capillary of 0.3 mm outer diameter and investigated at room temperature with a Bruker D8 Advance diffractometer using Cu-K α radiation (LynxEye 1-D detector, steps of 0.2°, acquisition time 7 s per step, Soller slits 2.5°, fixed divergence slit 8 mm, transmission geometry).The obtained products were phase pure according to X-ray powder diffractometry (Fig. 7).The structure models of Tb[H(PO 3 ) 4 ] and Ho[H(PO 3 ) 4 ] were confirmed and the structure model of Dy[H(PO 3 ) 4 ] was refined by Rietveld analysis 59,60 (Fig. 8) using the FullProf program suite and the WinPlotR graphical user interface. 61The single-crystal data of Tb[H(PO 3 ) 4 ] served as the starting structure model.The structure of Dy[H(PO 3 ) 4 ] was refined to excellent residuals of R p = 0.009, R wp = 0.012 and χ 2 = 4.04 (Table 11).The phase purity of Dy[H(PO 3 ) 4 ] could thus be confirmed.In Table 11 the data of the Rietveld refinement of Dy[H(PO 3 ) 4 ] are summarised.
The composition of Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy, Ho) was checked via EDX (Energy-dispersive X-ray) spectroscopy.For the doped samples Dy[H(PO 3 ) 4 ]:Ce and Dy[H(PO 3 ) 4 ]:Eu the ratios of Dy : Ce and Dy : Eu were also confirmed.Within the accuracy of the measurement limits no other elements were found.Single-crystals suitable for single-crystal X-ray diffraction analysis were selected under a polarizing microscope.Diffraction data were collected with a Bruker D8 Venture diffractometer using Mo-K α radiation (λ = 0.7093 Å) at a temperature of 293 ± 2 (Ln = Tb) and 296 ± 2 K (Ln = Ho), respectively.
The structure of the non-merohedrally twinned crystal of Tb[H(PO 3 ) 4 ] was solved by defining the twinned components using TWINABS (twin matrix: 0 0 1 0 −1 0 1 0 0; BASF = 0.430). 62Both crystal structures were solved by direct methods and refined by the full-matrix least-squares technique with the SHELXTL crystallographic software package. 63,64The Tb/Ho, P and O atoms could be clearly located.2][33] Relevant crystallographic data and further details of the structure determination are summarised in Table 5. Tables 6-9 show the atomic coordinates and displacement parameters.

Infrared spectroscopy
Infrared spectra were recorded at room temperature with a Bruker Equinox 55 FT-IR spectrometer using a Platinum ATR device with a scanning range from 4000 to 400 cm −1 .

UV/Vis spectroscopy
The optical reflection spectra were recorded with a Varian Cary 300 Scan UV/Vis spectrophotometer in the range of 200-800 nm.

Fluorescence spectroscopy
Excitation and emission spectra were recorded at room temperature with a Fluoromax-4 spectrofluorometer with a Xe plasma lamp (Horiba Scientific, Unterhaching).

Magnetic investigations
The temperature-dependent magnetic susceptibility data were recorded with a Quantum Design MPMS-XL super-conducting quantum-interference device (SQUID) magnetometer in the field of 1000 Oe between 1.8 K < T < 400 K.

Thermal analysis
The thermal analysis was carried out with a Netzsch STA-409 PC thermal analyzer in the temperature range of 25-1450 °C in air with a heating rate of 5 K min −1 .
10 Experimental section 10.1 Syntheses of Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy) Ln[H(PO 3 ) 4 ] (Ln = Tb, Dy) were synthesised via solid-state reactions starting from the respective lanthanide oxides and phosphorous acid.Tb 4 O 7 (94.70 mg, 0.14 mmol, Chempur, 99.9%) and Dy 2 O 3 (94.00mg, 0.25 mmol, Kristallhandel Kelpin, 99.99%), respectively, and H 3 PO 3 (264.10mg, 3.22 mmol, Sigma Aldrich, 99%) were ground in an agate mortar and transferred into a silica glass crucible.The reaction mixtures were covered with H 2 O 2 (20 ml, Merck, 30%) to suppress the formation of LnP 3 O 9 (Ln = Tb, Dy) as the side phase and heated up to 380 °C with a heating rate of 100 °C/h using a muffle furnace.The temperature was maintained for three hours before cooling down to room temperature by switching off the furnace.The products were washed with hot water and dried in air overnight.Different experiments showed that addition of H 2 O 2 to the reaction mixtures leads to phase pure samples of colourless Tb[H(PO 3 ) 4 ] and Dy[H(PO 3 ) 4 ].10.2 Synthesis of Ho[H(PO 3 ) 4 ]

Table 2
Absorption band energies for the strongest transitions in Tb[H(PO 3 ) 4 ] and in free Tb 3+ ions between 225 and 800 nm

Table 11
Rietveld refinement parameters of Dy[H(PO 3 ) 4 ] and thus the composition of Tb[H(PO 3 ) 4 ].Though Tb[H(PO 3 ) 4 ] only reveals a condensation in one dimension, it exhibits a remarkable thermal stability up to 500 °C before presumably H 2 O and P 2 O 5 are released.