New phase boundary and high-pressure thermoelasticity of ZnO

Abstract

The phase transition of zinc oxide from B4 (hexagonal wurtzite structure) to B1 (cubic rocksalt structure) has been studied by energy dispersive powder diffraction technique up to 11 GPa and 1273 K. Analysis of equation-of-state (PVT) data for the rocksalt phase yields precise values of the bulk modulus and its temperature derivative. The previously accepted P-T phase diagram is shown to be incorrect. It is established that the B1 phase is not recoverable. The equilibrium transition pressure of the B4-to-B1 transformation is near 6 GPa (at ambient temperature) and the dP/dT slope close to zero. These new results are confirmed by using simultaneously three other different types of experiment (imaging, ultrasonic and X-ray diffraction studies on single crystal specimens).

On the thermodynamic aspect of zinc oxide polymorphism: calorimetric study of metastable rock salt ZnO

“…The choice of P tr = 9.6 GPa based on averaging of ab initio estimations (that vary from 7.4 GPa [12] to 12.7 GPa [13]) and one randomly chosen value of the onset pressure of kinetically hindered phase transition at 300 K (that also varies from 7.5 GPa [7] to 10.0 GPa [14]). All this leads to completely wrong equilibrium P-T line in the high pressure -high temperature region, where the transition is governed by thermodynamics, and equilibrium experimental data are available [6,7].…”

“…Indeed, it is known that wurtzite ZnO starts to transform into rs-ZnO already at pressures above 5 GPa [6][7][8]. At room temperature this transition is very sluggish due to the existence of a strong kinetic barrier, which hinders the nucleation of a new phase [8].…”

“…At room temperature this transition is very sluggish due to the existence of a strong kinetic barrier, which hinders the nucleation of a new phase [8]. The transition pressure depends on temperature and shows a substantial (from 9 to 2 GPa at room temperature) hysteresis [6,7]. The hysteresis loop decreases with the temperature increase, and above 1000 K the branches of direct and reverse transitions merge at pressure of about 5.8 GPa, which can be considered as the equilibrium pressure of this phase transition.…”

“…Since the discovery of high-pressure polymorph of zinc oxide, rock salt ZnO in 1962 [1], its synthesis and properties have been intensively studied [2][3][4][5][6][7][8][9][10][11][12][13][14]. The most of these studies were conducted in situ under high pressure due to the problems with rs-ZnO recovery at ambient conditions.…”

High-pressure formation of MgxZn1−xO solid solutions with rock salt structure

“…In situ synchrotron radiation studies have shown that at room temperature the reversible transformation from wurtzite to rock salt phase in ZnO occurs at about 9 GPa [12][13][14][15][16][17]. Upon decompression down to 2 GPa, the rock salt phase reverts to the wurtzite one.…”