On the incommensurate phase in Ba Mn F₄: A neutron scattering study. Effect of the pressure

Abstract

A b s t r a c t : The incommensurate wave v e c t o r of t h e d i s t o r t i o n o c c u r r i n g i n BaMnF4 i s shown t o v a r y smoothly a r d p r e s e n t a t h e r m a l h y s t e r e s i s ; moreover, i t i s sample d e p e n d e n t . I t s b e h a v i o u r under p r e s s u r e i s d e t e r m i n e d up t o 11 Kbars, by p e r f o r m i n g i s o t h e r m i c and i s o b a r i c measurements. The r e s u l t s are c o n s i s t e n t w i t h a p i n n i n g o f t h e modulation by d e f e c t s . INTRODUCTION AND RECENT RESULTS

Incommensurate–Commensurate Transition in the Geometric Ferroelectric LaTaO4

“…It is noted that some previous studies of BaMnF 4 have misinterpreted the relationship between temperature and q , where a “Devil's staircase” was only revealed when studying single crystals of higher quality than that of previous investigations. [ 10 ] Powder methods obviously lack the same accuracy as analysis of a single crystal and so the seemingly continuous relationship seen in Figure 7a should be taken as an approximation of the temperature dependence of q . A characteristic jump between lock‐in values may not be evident in the current data and hence the possibility of a “Devil's staircase” should be investigated further in future work.…”

“…Guggenheim and co‐workers [ 8 ] showed it to involve a soft optical phonon branch at (0.392, 1/2, 1/2) and, rather uniquely among incommensurate phases, this modulation vector was found to be temperature independent. Later Scott and Blinc [ 9 ] suggested that this is the first step in a Devil's staircase at 5 a */13, and subsequent neutron studies [ 10 ] confirmed that there were indeed a series of such staircase jumps, which satisfy a formula that gives 5 a * at T lock‐in and 5 a */13 at the higher transition T IC . The presence of such a Devil's staircase was independently confirmed by several different kinds of experiment by Hidaka et al including X‐ray diffraction, [ 11 ] specific heat, [ 12 ] and piezoelectric resonance.…”