Subject classification: 61.50.Ks Scheelite-type AWO 4 binary oxides are important materials due to their use as scintillator detectors, photoanodes, solid laser hosts, and in optical fiber applications. They crystallize in the scheelite structure (SG: I4 1 /a, No. 88) [1], which may be regarded as a cubic close-packed array of A 2þ and [WO 4 ] 2-units with the coordination number of 8 and 4 oxygen atoms for the A and W cations, respectively. CaWO 4 and other tungstates have been studied by high-pressure Raman spectroscopy [2], the occurrence of pressure-induced structural transitions being detected. In addition, the room temperature (RT) phase behaviour of CaWO 4 has been studied by X-ray diffraction up to 65 GPa [3]. These diffraction studies have determined that a pressure-induced transition to the wolframite-type structure (SG: P2/c, No. 13) takes place at 12 GPa [3] and have also established that CaWO 4 becomes amorphous at pressure exceeding 40 GPa [3]. At high pressure and high temperature a new phase has been found in BaWO 4 [4] and PbWO 4 [5], having a characteristic dense structure. In this note, we report the observation of a quenchable new high-pressure and high-temperature phase of CaWO 4 at 45 GPa after heating to 477 K.
The existence of this new phase was verified by energy dispersive X-ray powder diffraction (EDXD), using a focused beam of 10 mm by 10 mm, and its structure was tentatively determined using the XRDA [6] and GSAS [7] program packages. These studies were carried out at the X-17C beam line at the National Synchrotron Light Source (NSLS) using a white beam and a Ge detector. The powder X-ray patterns were all collected at a diffraction angle of 2q ¼ 13 . CaWO 4 powder (99.78% purity) was compressed in a diamond anvil cell (DAC) to 45 GPa. No pressure medium was used in the experiments because it is known that alkaline earth tungstates are soft and can act as their own pressure medium [3,8]. A gold standard (99% purity), which was used for pressure calibration, was loaded along with the sample. We compressed the crystalline CaWO 4 sample until reaching the amorphization of it [3]. At 45 GPa CaWO 4 was already amorphized, but after heating the DAC to 477 K during two hours in an off-line electrical furnace, and then quenching to RT, the crystalline structure was recovered, being the final new phase we obtained. This is illustrated in Fig. 1, where a sequence of the diffraction spectra obtained at 2 GPa and 45 GPa at RT as well as at 45 GPa after heating is shown. After releasing pressure, no other changes that shift the peaks to lower energies were observed in the X-ray diffraction pattern indicating that the new phase remains stable at ambient pressure.
A typical X-ray diffraction pattern for the new phase is given in Fig. 2. The general characteristics of the powder pattern are quite different from those of the scheelite and wolframite phase [3]. Decomposition of CaWO 4 can be ruled out since similar tungstates are thermodynamically stable with respect to their component oxides (CaO and WO 3 in the present case) up to about 1200 K [4,5]. Then the X-ray pattern shown in Fig. 2 indicates the existence of a new CaWO 4 phase. The Bragg peaks of this new phase are broad. This is likely occurring because the sample is subject to significant non-hydrostatic stresses, due to the pressure medium used in the experiments, but may be also due to the presence of local disorders inherited from the amorphous phase. Under high pressure the AWO 4 compounds commonly assume monoclinic structures [2][3][4][5]. On the other hand, the only systematic absence among the reflections are in hkl for h þ k ¼ 2n þ 1. The space group is hence C2 (No. 5), Cm (No. 8) or C2/m (No. 12). Evidence indicating that the space group is indeed C2/m is given by the refinement, in which a monoclinic phase (SG: C2/m, No. 12) with a ¼ phys. stat. sol. (b) 231, No. 1, R1-R3 (2002)