ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2008, Vol. 72, No. 9, pp. 1297–1302. © Allerton Press, Inc., 2008. Original Russian Text © A.P. Kiselev, S.Z. Shmurak, V.V. Sinitsyn, S.S. Khasanov, B.S. Red’kin, A.V. Alekseev, E.G. Ponyatovskii, 2008, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2008, Vol. 72, No. 9, pp. 1367–1372. 1297 Spectroscopy and X-Ray Diffraction Analysis of Europium Molybdate Single Crystals Subjected to Different Thermobaric Treatments A. P. Kiselev a , S. Z. Shmurak a , V. V. Sinitsyn a , S. S. Khasanov a , B. S. Red’kin a , A. V. Alekseev b , and E. G. Ponyatovskii a a Institute of Solid-State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia b Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk, pr. Akademika Lavrent’eva 8, 630090 Russia e-mail: kiselev@issp.ac.ru; shmurak@issp.ac.ru Abstract—Europium molybdate (EMO) single crystals subjected to high uniform pressures (Pβ'-EMO) (expo- sure under 9 GPa at 300 K for 7 days) have been investigated by optical spectroscopy and X-ray diffraction. It is shown that Pβ'-EMO and amorphous europium molybdate have similar luminescence and transmission spec- tra. Annealing of EMO single crystals subjected to thermobaric treatments exhibits the same sequence of phase transitions as in the case of annealing of amorphous europium molybdate. It is shown that an EMO single crys- tal in the high-pressure state is a structurally inhomogeneous material consisting of two domains: a dominant amorphous-like part, in which the long-range order is completely absent, and a minor crystalline part. DOI: 10.3103/S1062873808090347 INTRODUCTION In recent years, the physical properties of amor- phous materials obtained by solid-phase amorphization (under uniaxial compression) have attracted the atten- tion of many researchers [1–5]. Radical changes in the spectral characteristics of europium molybdate Eu 2 (MoO 4 ) 3 (EMO) at a transition from the crystalline state to the amorphous state were found in [3, 4]. An unusual sequence of phase transitions was observed upon annealing of amorphous europium molybdate (a-EMO): first, at T ~ 550°C, a high-temperature β phase (β-Öåé) arises, which is thermodynamically stable at T > 881°C, and then a low-temperature α phase is formed at ~700°C, which is stable at T < 881°C. Sub- sequent annealing at T > 881°C returns the sample to the β modification. In [3, 4], to amorphize EMO, a fine-grained powder was prepared from the initial single-crystal sample and kept at a pressure of 9 GPa in a high-pressure chamber for 5 h. This treatment of polycrystalline samples led to their complete amorphization. At the same time, it was reported in [5] that a single- crystal EMO sample, subjected to uniform pressure of 9 GPa at 300 K for 7 days passes to a single-crystal high-pressure phase. Thus, the final state of EMO subjected to ther- mobaric treatment depends on its initial state. There- fore, it is expedient to perform comparative investiga- tions of the spectral and structural characteristics of polycrystalline and single-crystal EMO after ther- mobaric treatments. Such experiments were carried out in this study. 1. EXPERIMENTAL The investigations were performed on Eu 2 (MoO 4 ) 3 single crystals grown from a melt by the Czochralski method. The initial single crystals had a structure of the orthorhombic β' phase (sp. gr.Pba2). The thermobaric treatments of the samples were performed in a high- pressure chamber of toroidal type under uniform pres- sure of 9 GPa at 300 K for 7 days. An ethanol–methanol mixture, remaining in the liquid state at such pressures was used as a pressure-transferring medium. The sam- ples subjected to thermobaric treatments were 3 × 3 × 1 mm 3 in size and had (001), (010), and (100) facetings. X-ray diffraction analysis was performed in åÓK α radiation on a BRUKER X8 APEX diffractometer, equipped with a two-coordinate CCD detector, on small samples about 0.3 mm in size. Spectral investigations were performed on a system composed of two monochromators and a luminescence recording system, including a FEU-106 photomulti- plier and a wavelength scanner. The light source was a DKSSh-120 xenon lamp. An MDR-4 monochromator was used to study the luminescence excitation spectra of the samples in the wavelength range 275–500 nm (4.51–2.48 eV). An MDR-6 monochromator was applied to record the luminescence spectra of the sam- ples studied.