High pressure studies on α-cristobalite form of Al_0.5Ga_0.5PO₄: hydrostatic versus non-hydrostatic conditions

ABSTRACT

High pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al_0.5Ga_0.5PO₄. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C222₁ phase transforms to orthorhombic Cmcm phase at 4.9 GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19 GPa. The values of bulk modulus for C222₁ and Cmcm phases are 19(2) and 126(4) GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C222₁ phase to Cmcm phase has not observed up to 17.4 GPa. Instead, a new monoclinic phase P2₁ is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.

KEYWORDS:

• High pressure
• synchrotron
• X-ray diffraction
• Al_0.5Ga_0.5PO₄

Acknowledgements

The authors would like to thank Dr S.K. Kulshreshtha, BARC for the preparation of sample. We are also thankful to Dr Surninder M Sharma, BARC for his continuous encouragement and support during the study.

Disclosure statement

No potential conflict of interest was reported by the authors.