HYDROSTATIC-PRESSURE-TUNING FROM CLASSICAL TO TUNNELING MOTION AND FROM OFF- TO ON-CENTER POTENTIALS FOR Ag+, Cu+ AND Li+ DEFECTS

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Submitted on 1 Jan 1976

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HYDROSTATIC-PRESSURE-TUNING FROM

CLASSICAL TO TUNNELING MOTION AND FROM

OFF- TO ON-CENTER POTENTIALS FOR Ag+, Cu+

AND Li+ DEFECTS

U. Holland, F. Lüty

To cite this version:

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C7-272 JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 37, Dkcernbre 1976

HYDROSTATIC-PRESSURE-TUNING FROM CLASSICAL TO TUNNELING

MOTION AND FROM OFF- TO ON-CENTER POTENTIALS

FOR Ag+

,

Cuf

AND

Li+ DEFECTS

(*)

U. HOLLAND and F. LUTY

Physics Department, University of Utah, Salt Lake City, Utah, U. S. A.

Abstract. - The delicate balance among various defect-lattice interaction energies, producing off-center potentials for certain small point-defects, can be drastically upset by increasing the repul- sion energy through hydrostatic pressure application. Consequently, the multiwell off-center potential becomes more shallow and transforms eventually into a centro-symmetric single-well potential. This pressure tuning from off- to on-center behaviour has been studied for Ag+, Cuf and Li+ defects in various hosts through the temperature range 1.5 to 300 K. Dielectric loss measu- rements show for the classically reorienting Agf and Cu+ defects a gradual decrease of the reorien- tation barrier and eventually a change to tunneling behaviour under pressure, and for the quantum- mechanical KC1 : Li+ system a strong increase of the tunneling splitting. The size of the off-center dipole moment

<

p

>

was monitored by the real ( e cc

<

p > Z I T ) and integrated imaginary dielectric response, and - for Cu+ and Ag* ions - by the strength of the parity-forbidden d + s

transition which becomes partially allowed in off-center potentials. Both optical and dielectric data indicate a rather constant dipole moment, which collapses abruptly at a critical characteristic pressure in a way suggesting a local phase-transition picture for the off- to on-center symmetry change.

(*) Work supported by NSF Grant # DMR 74-13870/A 01.