Memory effects in pressure induced amorphousAlPO4

Abstract

A Brillouin scattering study of the pressure dependence of the elastic properties of AIPO4 single crystals is presented. On increasing the pressure there is clear evidence of a phase transformation at 15 GPa; this is consistent with previous Raman and x-ray measurements which report an amorphous phase at high pressures. On decreasing the pressure the sample not only recrystallizes at ^ 7 GPa but it reverts to a single crystal with the initial crystallographic orientation. This surprising behavior confirms the claims, based on birefringence measurements, of Kruger and Jeanloz [Science 249, 647 (1990)1. Our results also indicate that, even in the amorphous phase, AIPO4 remains elastically anisotropic.PACS numbers: 6l.50.Ks, 62.50.+p, 78.35,+c Since its relatively recent discovery [1], the phenomenon of pressure induced amorphization has received considerable attention [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The microscopic reasons behind this type of transformation are at present not yet well understood and are being actively investigated using various theoretical and computational modeling techniques [17][18][19][20][21]. Pressure induced amorphization has been found in insulators [1,[4][5][6][7][8][9][10][11][12][13][14], semiconductors [2,3,15], and metals [16]; it is therefore quite a general phenomenon. From the experimental side the effect is still somewhat controversial because it relies on the vanishing of xray or Raman peaks in the experimental spectra and it always can be argued that their disappearance is due to insufficient sensitivity. Phenomenologically the materials can also be classified into two groups, one in which the materials retain their amorphous structure on pressure release and another in which they recrystallize. A similar phenomenon, pressure release amorphization, has also been discovered relatively recently [22].It has been recently shown by Kruger and Jeanloz [10] that AIPO4 belongs to the class of crystals that recrystallize on pressure release. Furthermore, based on the fact that recovered samples had the same birefringence as initial samples, the authors also concluded that the recrystallized samples had the same crystallographic orientation as the starting material and that, in the amorphous phase, it must retain a "memory" of the crystalline phase. The existence of a memory effect was subsequently observed in molecular dynamics calculations [20]. Attempts to further understand this process have concentrated on the sister compound S1O2 quartz which has the same structure (replacing the Al and P atoms by Si) but does not recrystallize on pressure release. Because S1O2 does not recrystallize it allows experiments to be made at atmospheric pressure, greatly simplifying the measurements. The amorphous nature of samples of pressurized SiC>2 was originally ascertained using x-ray techniques [23]. Initial transmission electron microscopy (TEM) measurements [24] up to 30 GPa showed no evidence of crystallinity and no difference from ordinary "fused" silica. More recent TE...

Pressure‐induced structural phase transitions in materials and earth sciences

“…These results indicate that arsenates undergo phase transitions where an increase of As coordination from four to six is observed and that the observation of amorphisation is related to high kinetic energy barriers of the first-order phase transitions that cannot be overcome at RT. A similar increase of P coordination in AlPO 4 and GaPO 4 had not been observed under pressure [265,266], and the pressure-induced amorphisation in AlPO 4 above 15 GPa showed a memory effect and was reversible unlike in other compounds [267][268][269]. A similar memory effect of amorphous phases is also observed in the collapsing of high crystobalites PBO 4 and AsBO 4 [270].…”

Optical Spectroscopy Methods and High‐Pressure–High‐Temperature Studies

“…Amorphization was detected by energy dispersive X-ray diffraction on a powder, whereas the reversibility and recovering of the orientations was detected by birefringence measurements on a single crystalline sample, with measurements at ambient conditions, before and after a pressure run to 15 GPa. It was proved by Brillouin scattering under pressure [32] that the transition was indeed reversible with the same crystalline orientation before and after pressurization, and moreover that the high-pressure phase was elastically anisotropic. The first proof that the highpressure phase was not amorphous was given by a Raman study [33] where it was shown that above 14 GPa, there were narrow Raman lines, footprint of a crystallinity.…”

“…32 Phase transition points of CO 2 from the Raman scattering (Reprinted figure from Ref [107]. by permission from EDP Sciences).…”

Origin of “memory glass” effect in pressure-amorphized rare-earth molybdate single crystals

“…Recovery of original optical birefringence of bulk single crystal samples after high pressure treatment was interpreted as evidence of the MGE: the compressed amorphous state had to retain "memory" of the initial crystalline state and orientation. This work was followed by numerous experimental as well as theoretical studies of both pressure-induced amorphization (PIA) and closely related MGE in berlinite and other materials [2][3][4][5][6][7][8][9][10]. Some of them have raised doubts about the high pressure amorphization of AlPO 4 because transition from the α-phase to a poor crystalline phase was observed rather than transition to a purely amorphous state [10,11].…”