Pressure and temperature-dependent optical properties of TiTa2O7

Polycrystalline TiTa2O7 was synthesized directly by solid-state reaction methods. The structures were determined by using X-ray diffraction (XRD). The high-pressure Raman and UV-vis absorption spectra of TiTa2O7 were obtained up to 25 GPa in a diamond anvil cell (DAC) at room temperature. The Raman scattering results reveal that a pressure-induced amorphization occurs above 10.5 GPa. An inflection point was also observed at 11 GPa in the pressure-dependent bandgap energy spectra, which agrees well with the amorphization point found in Raman spectra. The temperature-dependent Raman and photoluminescence (PL) spectra of TiTa2O7 were also measured. The PL mechanism for TiTa2O7 was studied. It is worth noting that the Raman vibrations attributing to the bending vibration of the Ta (Ti)–O octahedron exhibit anomalous frequency shifts in both the high-pressure and temperature-dependent Raman spectra.


Introduction
Tantalum oxides have short metal-oxygen bond distances, high-temperature resistance and good chemical resistance, as well as excellent optical properties and expected high hardness, 1 which have obtained worldwide attention. As a typical tantalum oxide, titanium tantalate (TiTa 2 O 7 ) has a high average refractive index, relative high hardness, and high temperature resistance, these materials could be of interest for further potential applications, including synthetic gemstones, optical coatings, or lowthermal-expansion materials. 1 In recent years, the tantalates and niobates with titanium were found to have excellent photocatalysis, for example: TiNb 2 O 7 (ref. 2) and NaTaO 3 . 3 Tantalate photocatalysts have unique electronic and band structure and their catalytic performance is better than other types of compounds in the decomposition of water to hydrogen, good examples are Ti-Ta alloy-based nanotubular oxide 4 and mesoporous Ta 2 O 5 -TiO 2. (ref. 5) TiTa 2 O 7 , the focus of our research is also considered as a promising photocatalyst. 4 Its defects with a slightly wide band-gap under ambient conditions are expected to be modulated at high pressure. TiTa 2 O 7 crystallizes is a monoclinic structure (space group C2/m), its lattice parameters are a ¼ 20.351(3) A, b ¼ 3.801(2) A, c ¼ 11.882(2) A, and b ¼ 120.19 (1) . The structure of TiTa 2 O 7 can be described as '3 Â 3 shear ReO 3 ' structure, consisting of TiO 6 octahedra sharing corners and edges (Fig. 1). This structure contains fragments of the ReO 3 structure in the form of blocks of corner-sharing MO 6 octahedra (M ¼ Ti, Ta). Each of these blocks contains nine MO 6 (3 Â 3) octahedra forming linear columns along the b-axis. Perpendicular to the b-axis, the columns are connected by crystallographic shear planes. MO 6 octahedra share edges across these shear planes. 6 There is no ordering of the Ti 4+ and Ta 5+ cations among different crystallographic sites in the structure of TiTa 2 O 7 . 7 Niobates and tantalates are isostructural with the same oxygen occupancy. Only a few studies were carried out caring about ANb 2 O 6 (A ¼ Fe, Mn, Mg) with columbite structure, [8][9][10] and have indicated a pressure-induced volume decrease and distortion of the AO 6 and NbO 6 octahedra. However, the inuence of the octahedral shape on properties and the monoclinic structure (TiTa 2 O 7 ) under high-pressure is still unknown.
Herein, we have synthesized polycrystalline TiTa 2 O 7 , and its property is investigated using high pressure Raman spectroscopy and UV-vis absorption in DAC, as well as the temperaturedependent Raman and photoluminescence (PL). Above 10.5 GPa, monoclinic titanium tantalate started amorphization and completed above 17 GPa.

Experimental
TiTa 2 O 7 was synthesized by the conventional high-temperature solid-state reaction method. Stoichiometric amounts of TiO 2 (Alfa Aesar, 99.995%) and Ta 2 O 5 (Alfa Aesar, 99.99%) were mixed and ground well, the reactant mixtures were rammed and heated in alumina crucibles. The melting temperature for the TiTa 2 O 7 is 1948 K, the pipe furnace was used to heat the mixtures gradually from 1623-1723 K with intermittent grindings. The XRD patterns of synthesized TiTa 2 O 7 (Fig. 2) were recorded at room temperature using a Rigaku D/max-r A 12 kW X-ray diffractometer with Cu Ka radiation (l ¼ 1.5406 A) operated at a current of 40 mA and a voltage of 40 kV. The XRD patterns t well with the standard card (JCPDF (211424)). The result shows that the sintering product of TiTa 2 O 7 is of high purity.
The morphology of as-prepared samples before and aer compression was characterized by scanning electron microscopy (SEM, 15 kV, FEI Magellan 400).
Confocal Raman spectra of TiTa 2 O 7 were excited using the 532 nm emission line of a frequency-doubled Nd:YAG laser with a Horiba Jobin Yvon Labram-HR Evolution Raman spectrometer. The spectra were collected unpolarized under ambient conditions in a back-scattering geometry. The pressure was calibrated using the ruby uorescence method. 11 The T301 stainless gasket was pre-indented to a thickness of 56 mm, and a hole of 180 mm in diameter was drilled in the centre of the gasket, the TiTa 2 O 7 powder was load in the sample chamber of DAC with the thickness of 30 mm and 70 Â 150 mm in size. The schematic diagram is shown in Fig. 3. We injected argon as the pressure transmitting medium (PTM).
The temperature-dependent PL and Raman spectra were taken with a Jobin-Yvon HR800 micro-Raman spectrometer and an argon ion laser (514.5 nm) was served as the excitation source. The temperature of the sample was adjusted by the THMSE 600 Temperature Programmator (Linkam Scientic Instruments) attached to the Raman spectrometer in a range of 183.15 to 803.15 K.
The UV-vis absorption spectra were recorded using a UV-vis spectrophotometer (Perkin Elmer Lambda 850) with a synthetic IIa type diamond possessing a high transmittance in the UV region and permit photoemission studies. A deuteriumhalogen (250-1000 nm) light source and an Ocean Optics QE65000 spectrometer were used as the excitation source and detector, separately.

High-pressure Raman spectra
The Raman spectrum of TiTa 2 O 7 at ambient condition is shown in Fig. 4, and the respective Raman modes are indexed. Based on previous studies, 1,12 we attribute the Raman peak in the lowwavenumber region (<150 cm À1 ) to external modes belonging to Ta-Ta vibrations, only metal-metal vibrations (n 1 ) will occur in this region. Raman modes between 150 and 400 cm À1 can be probably assigned to O-Ti-O or O-Ta-O symmetric and antisymmetric bending vibrations (n 2 ). Besides, Ti-O vibrations should be visible at lower frequencies than Ta-O vibrations, but   only if the metal atoms participate in the vibration. According to Eror and Balachandran, 12 the metal-oxygen vibrations (n 3 ) of the TiO 6 octahedra occur in the wavenumber region between 550 and 700 cm À1 , whereas two bands at 899 and 1020 cm À1 can be assigned to the symmetric metal-oxygen stretching vibrations (n 4 ) of the corner-and edge-sharing TaO 6 octahedra, respectively.
To conrm the structural evolution and phase stability of TiTa 2 O 7 under high pressure, the high-pressure Raman spectra of TiTa 2 O 7 were performed up to 25.17 GPa. For comparison, representative Raman spectra of TiTa 2 O 7 at various pressures upon compression and decompression are shown in Fig. 5a. In low-pressure realm, little change is observed until 3.75 GPa, where the 75 cm À1 mode split into two peaks. Above 10.5 GPa, the original peaks from the monoclinic TiTa 2 O 7 collapsed into two very broad bands at about 100 and 650 cm À1 , indicating the pressure-induced amorphization was taken place. Aer releasing the pressure, no Raman mode was recovered, which implies that the pressure-induced transformation was completely irreversible. These results were conrmed by multiple experiments with different compressing rates, no difference was found when we retain pressure in a relatively long period. While we change the power of the laser, the wavenumber of titanium tantalate didn't suffer a signicant change, the intensity of the peak went with corresponding laser power with a certain coefficient.
To further understand the high-pressure behavior of TiTa 2 O 7 under high pressure, Raman frequencies of TiTa 2 O 7 as a function of pressure are plotted in Fig. 5b, and the corresponding data are shown in Table 1. It is found that most of the vibration modes shi to higher frequencies with pressures until 10.51 GPa. Usually, the vibrational mode frequencies are expected to increase as bonds are compressed. However, 182 cm À1 , 162 cm À1 , 279 cm À1 , and 297 cm À1 modes of n 2 are found to shi towards lower wavenumbers with increasing pressure. The existence of such so modes indicates the elongation of the O- Ta The SEM images of TiTa 2 O 7 were taken before and aer the compression. It could be clearly observed that the porosity of the sample were decreased aer it was recovered to ambient conditions, due to the relative high hardness, no signicant change of grain size were found.
Because of its high symmetry, TaO 6 octahedra has only one independent bond angle, and the other bond angles change with this independent one. 13 It can be considered that the shortening of six Ta-O bonds in different degrees is accompanied by the change of O-Ta-O/O-Ti-O bond, moreover, the bond length of Ta-Ta, corresponding to the Raman located at 75 cm À1 , became longer as the pressure increased, in other words, the distance between the two TaO 6 octahedra became further, while the volume of the TaO 6 octahedra became smaller, which leads to the distortion and deformation of bonds between the TaO 6 octahedra. Eventually, the TiTa 2 O 7 framework ultimately collapsed into the amorphous state. The XRD pattern were measure aer decompression as shown in Fig. 6, the characterize peak of TiTa 2 O 7 were disappeared and the remaining peak t the standard card of TiO (JCPDF (231078))  well, indicating that the sample entered the amorphous phase and the transition is irreversible.

UV-vis absorption spectra under high pressure
To shed light on the complex pressure-induced order-disorder processes occurring in TiTa 2 O 7 and to understand the relationship between structure and optical properties, UV-vis absorption spectroscopy was performed and focused on the explanation of the strong nonlinear pressure dependence of the direct bandgap energy in TiTa 2 O 7 at relatively low pressures. Fig. 7a plots the pressure-dependent absorption spectra of TiTa 2 O 7 . The TiTa 2 O 7 has a low absorption coefficient (a) (about 0.25 cm À1 ) in the wavelength range from 1000 to 425 nm, representing the transparent region. As for the ultraviolet region (<425 nm), the crystal has heavy absorption.
Photon energy hn can depend on the absorption coefficient by the following eqn (1): 14,15 ahn where k can be 1/2, 2, 3/2, or 3 to allow direct, indirect, forbidden direct, and indirect transitions, respectively. For the TiTa 2 O 7 , the best k tted to be 1/2, which demonstrates that TiTa 2 O 7 is a direct transition material, and it also agrees well with previous theoretical results. 1 The band-gap energy was achieved to be 3.40 eV by extrapolating the linear region of (ahn) 2 vs. hn, as shown in Fig. 7b, which ts well with previous study. 2,4 The sample maintained a relatively clear absorption edge within the pressure range of 0-12 GPa. It can be observed that the direct bandgap energy of TiTa 2 O 7 exhibits a strong nonlinear pressure dependence up to 15 GPa. Therefore, we infer that pressure leads to the valence band maximum (VBM) and the conduction band minimum (CBM) change of the band structure. Generally, upon compression, the lattice constant and interatomic distances become smaller, the wave functions become more overlapping, this led to the broadening of the bandwidth, as a result, the band-gap became narrower. 16 From the high-pressure Raman spectrum of TiTa 2 O 7 , the elongation of the O-Ta-O/O-Ti-O bond with increasing pressure is a good t for the fact that in Fig. 7c, the direct bandgap energy in TiTa 2 O 7 increases within the range 0-7.3 GPa, and decrease aer 7.3 GPa. It can be explained that under 10.5 GPa, the volume of the TaO 6 octahedra decreases gradually, and the interatomic spacing corresponding to the vibration mode in the high wavenumber region became smaller with the increase of pressure. And there is an inection point in the decreasing trend at 11 GPa, similarly, the previous Raman spectroscopy shows that above 10.51 GPa, the amorphous phase transition of TiTa 2 O 7 began to take place.
Our results indicate that the nonlinear pressure dependence of the direct bandgap energy is related to the bending vibration of n 2 mode at high pressure, the inection point at 11 GPa could be mainly ascribed to the effect of TiTa 2 O 7 amorphization.

Temperature-dependent Raman spectra
To understand the variable-temperature behavior in TiTa 2 O 7 at ambient pressure, in situ temperature-dependent Raman measurement of TiTa 2 O 7 crystal was carried out in temperature range from 83.15 to 803.15 K. The spectra of TiTa 2 O 7 was collected every 42 K. The Raman spectra of all temperature points are shown in Fig. 8a. It can be seen from the spectra that   with the increase of temperature, the Raman peak shis to lower wavenumber, and the peak width widens continuously. No sudden change of peak position or new Raman peak can be observed, and there is no phase transition occurs in the whole temperature range. With the increasing temperature, the thermal motion of atoms in the polymer increases gradually, while the force between atoms decreases, which leads to the Raman peaks shi towards lower wavenumber. The irregular thermal motion of atoms is the reason for abnormal vibration of each bond together with the decrease of molecular structure order, which nally contributes to the gradual broadening of the superposition of similar Raman vibration frequencies.
Former research indicates that, at higher temperatures, the maximum thermal expansion occurs perpendicular to the endless linear columns and zigzag chains of the corner-and edge-sharing octahedra, respectively. Along the crystallographic b-axis, negative thermal expansion values can be partly observed in the previous study. 1 Raman mode frequencies of TiTa 2 O 7 as a function of temperature are plotted in Fig. 8b, and the corresponding data are shown in Table 1. In Fig. 8b, the Raman peaks 222 cm À1 and 262 cm À1 modes of n 2 are found to shi towards lower wavenumbers with the decrease of temperature, this may be related to the bending vibration of n 2 mode at high pressure. We have reason to believe that the negative thermal expansion of TiTa 2 O 7 along the b-axis at a high temperature can be explained by the change of the bond angle of the O-Ta-O/O-Ti-O bonds by the external force under the condition of increasing pressure or decreasing temperature.

Temperature-dependent PL spectra
At ambient conditions, the PL of TiTa 2 O 7 is relatively weak compared with other materials, so no PL could be seen when it was placed in a DAC. Generally speaking, decreasing temperature and increasing pressure may induce a similar effect on studied samples, thus the temperature depend PL was collected here. Fig. 9a shows the PL spectrum of TiTa 2 O 7 at 83.15 K, the PL peak can be well tted (Gaussian prole) to two emission peaks which centred at 1.42 eV and 1.51 eV. Notably, the PL emission centres at 1.42-1.51 eV and instead of the obtained bandgap ($3.4 eV) from TiTa 2 O 7 excited by using a 514 nm laser line at 83.15 K, this phenomenon cannot be attributed to the normal band-to-band transition. Since the electronic conguration of TiTa 2 O 7 is the same as that of niobate-oxygen octahedral, its luminescence mechanism is consistent with that of TiTa 2 O 7 . In the case of PbMg 1/3 Nb 2/3 O 3 -PbIn 1/2 Nb 1/2 O 3 systems (PMN-PIN), the special luminescence tend to associate the origin of PL bands with the Nb-O systems without the inuence of Mg and Pb. 17 The two tting PL peaks at 1.42 and 1.51 eV of TiTa 2 O 7 , consistent with the A peak (with a stronger intensity and higher energy) and B peak (with a weaker intensity and lower energy), which are mainly determined by the defect and regular Ta-O respectively. To understand the observed phenomenon, we have to consider the non-radiative transition of the exciton. The photo-excitation allows electrons (e À ) in the oxygen states above the valence band (V o ) to the conduction band (CB) and leaving holes in the V o , and then the excited (e À ) in the CB relaxes to intrinsic Ta i aer a non-radiative transition process. Eventually, the relaxed (e À ) recombines with holes cause the PL emission. There would be two types possible in this case, one is the B peak, another is the A peak which is led by defect states. Therefore, the defect emission A is from the e À recombines with holes directly. While the B procedures an emission and vibration, relax photon and phonon.
The temperature dependence of TiTa 2 O 7 PL is shown in Fig. 9b, the intensity decreased when the in situ temperature rose. The PL intensity is sensitive to temperature and can be easily affected. The rise of temperature oen leads to a decrease in PL intensity, the main reason is the internal energy conversion of the molecule. As the temperature increased, the molecular thermal motion became more intense, resulting in weaker interatomic bonding and electronic structure became distorted, all of these consequences led to uorescence quenching. And the temperature-dependent PL emission can also be tted by Gaussian to two peaks. The temperaturedependent value of the two peak centres from 83 to 283 K is plotted in Fig. 9c. As the temperature increased, both of the two centres blue-shied and we tted the two data linearly. The shi speed of defect A bonds (1.99 Â 10 À4 eV K À1 ) is slightly faster than the B bonds (1.45 Â 10 À4 eV K À1 ). The blueshis can be attributed to the shrinkage of Ta i -CB and V o -VB, and the result of the broaden of recombining energy between Ta i and V o .

Conclusions
The optical properties of TiTa 2 O 7 under high pressure and different temperature are reported in this article respectively. In the high-pressure Raman spectra, the TiTa 2 O 7 was amorphized at 11 GPa. The anomalous frequency shi in the high-pressure and temperature-dependent Raman spectra, as well as the nonlinear pressure dependence of the direct bandgap energy can be attributed to the bending vibration of the O-Ta-O and O-Ti-O bonds in the Ta (Ti)-O octahedra and negative thermal expansion coefficient along the b-axis of the crystal structure. The PL spectra of TiTa 2 O 7 , attributed to the TaO 6 octahedron, are also presented. These results are very important for understanding the high-pressure and low-temperature behaviors in tantalates for their applications in extreme environments.

Conflicts of interest
There are no conicts to declare.