Room temperature angle-dispersive x-ray diffraction measurements on spinel ZnGa2O4 up to 56 GPa show evidence of two structural phase transformations. At 31.2 GPa, ZnGa2O4 undergoes a transition from the cubic spinel structure to a tetragonal spinel structure similar to that of ZnMn2O4. At 55 GPa, a second transition to the orthorhombic marokite structure (CaMn2O4-type) takes place. The equation of state of cubic spinel ZnGa2O4 is determined: V0 = 580.1(9) A3, B0 = 233(8) GPa, B0'= 8.3(4), and B0''= -0.1145 GPa-1 (implied value); showing that ZnGa2O4 is one of the less compressible spinels studied to date. For the tetragonal structure an equation of state is also determined: V0 = 257.8(9) A3, B0 = 257(11) GPa, B0'= 7.5(6), and B0''= -0.0764 GPa-1 (implied value). The reported structural sequence coincides with that found in NiMn2O4 and MgMn2O4.
Deep Dive into Post-spinel transformations and equation of state in ZnGa2O4: Determination at high-pressure by in situ x-ray diffraction.
Room temperature angle-dispersive x-ray diffraction measurements on spinel ZnGa2O4 up to 56 GPa show evidence of two structural phase transformations. At 31.2 GPa, ZnGa2O4 undergoes a transition from the cubic spinel structure to a tetragonal spinel structure similar to that of ZnMn2O4. At 55 GPa, a second transition to the orthorhombic marokite structure (CaMn2O4-type) takes place. The equation of state of cubic spinel ZnGa2O4 is determined: V0 = 580.1(9) A3, B0 = 233(8) GPa, B0’= 8.3(4), and B0’’= -0.1145 GPa-1 (implied value); showing that ZnGa2O4 is one of the less compressible spinels studied to date. For the tetragonal structure an equation of state is also determined: V0 = 257.8(9) A3, B0 = 257(11) GPa, B0’= 7.5(6), and B0’’= -0.0764 GPa-1 (implied value). The reported structural sequence coincides with that found in NiMn2O4 and MgMn2O4.
Cubic oxide spinel AM 2 O 4 compounds (A: bivalent cation and M: trivalent cation) occur in many geological settings of the Earth's crust and mantle, as well as in lunar rocks and meteorites. The study of their high-pressure structural properties is important for improving the understanding of the constituents of the Earth. Highpressure studies have been performed in MgM 2 O 4 spinels (e.g. MgAl 2 O 4 ) revealing that upon compression they may adopt orthorhombic CaFe 2 O 4 -, CaMn 2 O 4 -, or CaTi 2 O 4 -type structures [1]. However, the structure and properties of post-spinel phases is presently still under debate. On top of MgM 2 O 4 spinels, the high-pressure properties of ZnM 2 O 4 cubic spinels (e.g. ZnAl 2 O 4 ) have been studied too. Among them, ZnAl 2 O 4 [2] and ZnFe 2 O 4 [3] have been experimentally investigated. The first one remains stable up to 43 GPa in the cubic spinel structure but the second one transforms to either a CaFe 2 O 4or a CaTi 2 O 4 -type structure beyond 24 GPa. In addition to these facts, in other compounds like AMn 2 O 4 spinels, cubic-to-tetragonal transitions have been reported to occur at pressures as low as 12 GPa [4]. In contrast with the materials above mentioned, the high-pressure structural stability of AGa 2 O 4 spinels has not been studied yet. In order to shed more light on the understanding of the high-pressure properties of AM 2 O 4 cubic spinels, we report a study of the high-pressure structural properties of zinc gallate (ZnGa 2 O 4 ) up to 56 GPa. The present research work contributes to achieve a fuller understanding of how cation replacement affects the high-pressure behavior of oxide spinels.
ZnGa 2 O 4 powders were synthesized by a solid state reaction at high temperature by mixing appropriate quantities of ZnO and Ga 2 O 3 precursors and firing at 1100ºC for 24 h [5]. Chemical and structural analyses have shown the stoichiometric composition of ZnGa 2 O 4 and the presence of traces of impurities ( -Ga 2 O 3 , less than 1% by volume, and ZnO, less than 0.2% by volume). Angle-dispersive x-ray diffraction (ADXRD) experiments were carried out at room temperature (RT) at high pressure up to 56 GPa at Sector 16-IDB of the HPCAT -Advanced Photon Source (APS) -using a diamondanvil cell (DAC) and an incident monochromatic wavelength of 0.36816(5) Å. Samples were loaded in a 100 m hole of a rhenium gasket in a Mao-Bell-type DAC with diamond-culet sizes of 300 m. Ruby grains were loaded with the sample for pressure determination [6] and silicone oil was used as pressure-transmitting medium [7,8]. The monochromatic x-ray beam was focused down to 20 20 m 2 using Kickpatrick-Baez mirrors. The images were collected using a MAR345 image plate located at 350 mm from the sample. They were integrated and corrected for distortions using FIT2D. The structural analysis was performed using POWDERCELL.
At ambient pressure the wide band-gap semiconductor ZnGa 2 O 4 has a cubic spinel structure (space group: From the refinements we obtained the evolution of the unitcell and oxygen position parameters as a function of pressure. Fig. 2 shows the pressure dependence of the volume (only the squares correspond to c-ZnGa 2 O 4 ). A fit to the data reported up to 25.7 GPa with a third-order Birch-Murnaghan equation of state (EOS) [9] gives: V 0 = 580.1(9) Å 3 , B 0 = 233(8) GPa, and B 0 ´= 8.3(4), where V 0 , B 0 , and B 0 ’ are the zero-pressure volume, bulk modulus, and its pressure derivative, respectively.
According with these parameters the implied value of the second derivative of the bulk modulus versus pressure at ambient conditions is B 0 ‘’= -0.1145 GPa -1 . The EOS fit is shown as a solid line in Fig. 2 and the agreement with the experiments is found to be good. According to this result ZnGa 2 O 4 is the less compressible oxide spinel among those studies up to now (see Table I) [2, 3, 11 -16]. On the other hand, the estimated B 0 is in good agreement with the theoretical predictions made using the self-consistent tight-binding linearized muffin-tin orbital (LMTO) method [17,18], the ab initio perturbed ion (aiPI) model [19,20], or the local-density approximation (LDA) [21,22].
All these theoretical methods give values for B 0 ranging from 207 to 243 GPa, which include our experimental value (see Table II). In contrast with other theoretical methods, the generalized gradient approximation (GGA) gives an underestimated value for B 0 [22,23] as can be seen in Table II. It should be said here, that a partial inversion between Zn and Ga sites could cause a decrease of the compressibility of the ZnO 4 tetrahedra. A decrease of this compressibility would necessary lead to an enhancement of B 0 if we compare it with that of the normal spinel [19]. Evidence of such pressureinduced cation inversion has been found in other oxide spinels upon compression; e.g.
NiAl 2 O 4 [24]. However, as we discussed above, inversion cannot be detected in ZnGa 2 O 4 due to the similar x-ray scatter
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