 
Summary: VOLUME 78, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 30 JUNE 1997
Phonon Softening and Elastic Instabilities in the CubictoOrthorhombic
Structural Transition of CsH
A. M. Saitta,1,
* D. Alfè,1,
* S. de Gironcoli,1,
* and S. Baroni,1,2,
*
1
INFMIstituto Nazionale per la Fisica della Materia and SISSAScuola Internazionale Superiore di Studi Avanzati,
Via Beirut 24, I34014 Trieste, Italy
2
CECAMCentre Européen de Calcul Atomique et Moléculaire, ENSL, 46 Allée d'Italie, 69364 Lyon Cedex 07, France
(Received 21 October 1996)
The cubictoorthorhombic structural transition occurring in CsH at a pressure of about 17 GPa is
studied by ab initio calculations. The relative stability of the competing structures and the transition
pressure are correctly predicted. We show that this pressureinduced firstorder transition is intimately
related to a displacive secondorder transition which would occur upon application of a shear strain to
the (110) planes. The resulting instability is rationalized in terms of the pressureinduced modifications
of the electronic structure. [S00319007(97)034625]
