The P21/m<->C2/m displacive phase transition has been studied on a series of synthetic amphibole with ideal unit formula ANa1-y B(Na1+zMg1-z) C(Mg5-xR3+x) TSi8 O22 XF2 where x = y + z and R3+ is Ti, V, and Ga. These data were compared with those obtained for ANa0.83 B(Na0.83 Mg1.17) CMg5 TSi8 O22 X(OH)2 (Cámara et al. 2003), and allowed understanding of how the size of octahedral cations and the F1 OH-1 exchange affect the critical temperature (Tc) and the energetic parameters of the second-order transition. The crystals are very small, and are often twinned. XRD analysis was done at room T with a Bruker AXS SMART APEX diffractometer equipped with a monocapillary glass guide to enhance X-ray intensity. XRD analysis at non-ambient conditions was done with a Philips PW1100 diffractometer. Weighted least squares refinements were done on all the independent reflections for the P21/m phase and on those with I > 3?I for the C2/m phase. In the case of ANa0.90 B(Na0.92Mg1.08) C(Mg4.98V0.02) TSi8 O22 XF2, which mostly witness the effects of the F1 OH-1 exchange, changes in unit-cell parameters and in the intensities of the selected set of super-lattice reflections were monitored in the T range 298-944 K. Polynomial fitting of a 24 Landau potential to the evolution of the order parameter with T yielded a Tc of 391±3 K, 139 K lower than in the (OH)2 counterpart. Single-crystal structure refinements done at 298, 406, 514, 621, 729, 836 and 944 K suggest that changes in Tc mainly depend on the differential thermal expansion of the B-group and C-group sites, the latter being strongly related to the occurrence of fluorine at the O3 site. The behaviour of the other two compositions, where the contents of trivalent cations are Ga0.13 and Ti3+0.08, allows understanding of the contribution of all the group-sites to the progressive lowering of the Tc, which actually approaches room T. The presence of fluorine is also responsible for significant differences in the ordering of Na at the A-group sites, because it avoids the ANa-OH hindrance, and allows a further shift of Na towards the basal oxygen atoms of the tetrahedra. The A cations in P21/m amphiboles occur at the x ¼ z position, and the electron density at the A site shows a single peak. In C2/m amphiboles, the A cations may distribute themselves between two special positions (x ½ z and 0 y 0) which both repeat themselves within the A cavity. Therefore, the electron density at the A site is built up by 2 to 4 coalescing peaks; C2/m fluoro-amphiboles generally have Na strongly ordered at the x ½ z position, and show two more or less linked peaks. When approaching the transition, we found clear evidences of an incipient loss of the 21 local symmetry. This process can be followed monitoring the changes in shape of the difference Fourier peak, which in turn can be used together with the increasing similarity of the two independent O5-O6-O5 angles as a structural indicator of the approaching transition. Because the results of X-ray diffraction analysis are averaged over the whole crystal volume, both these features may also indicate that the transition proceeds rather continuously at slightly different T in different crystal domains. Cámara, F., Oberti, R., Iezzi, G., and Della Ventura, G. (2003) The P21/m<->C2/m phase transition in synthetic amphibole Na NaMg Mg5 Si8 O22 (OH)2: thermodynamic and crystal-chemical evaluation. Physics and Chemistry of Minerals, 30, 570 581.
The P21/m ? C2/m phase-transition in amphiboles: the effects of the size of octahedral cations and of the F1OH-1 exchange and crystal-chemical changes at the transition
2004
Abstract
The P21/m<->C2/m displacive phase transition has been studied on a series of synthetic amphibole with ideal unit formula ANa1-y B(Na1+zMg1-z) C(Mg5-xR3+x) TSi8 O22 XF2 where x = y + z and R3+ is Ti, V, and Ga. These data were compared with those obtained for ANa0.83 B(Na0.83 Mg1.17) CMg5 TSi8 O22 X(OH)2 (Cámara et al. 2003), and allowed understanding of how the size of octahedral cations and the F1 OH-1 exchange affect the critical temperature (Tc) and the energetic parameters of the second-order transition. The crystals are very small, and are often twinned. XRD analysis was done at room T with a Bruker AXS SMART APEX diffractometer equipped with a monocapillary glass guide to enhance X-ray intensity. XRD analysis at non-ambient conditions was done with a Philips PW1100 diffractometer. Weighted least squares refinements were done on all the independent reflections for the P21/m phase and on those with I > 3?I for the C2/m phase. In the case of ANa0.90 B(Na0.92Mg1.08) C(Mg4.98V0.02) TSi8 O22 XF2, which mostly witness the effects of the F1 OH-1 exchange, changes in unit-cell parameters and in the intensities of the selected set of super-lattice reflections were monitored in the T range 298-944 K. Polynomial fitting of a 24 Landau potential to the evolution of the order parameter with T yielded a Tc of 391±3 K, 139 K lower than in the (OH)2 counterpart. Single-crystal structure refinements done at 298, 406, 514, 621, 729, 836 and 944 K suggest that changes in Tc mainly depend on the differential thermal expansion of the B-group and C-group sites, the latter being strongly related to the occurrence of fluorine at the O3 site. The behaviour of the other two compositions, where the contents of trivalent cations are Ga0.13 and Ti3+0.08, allows understanding of the contribution of all the group-sites to the progressive lowering of the Tc, which actually approaches room T. The presence of fluorine is also responsible for significant differences in the ordering of Na at the A-group sites, because it avoids the ANa-OH hindrance, and allows a further shift of Na towards the basal oxygen atoms of the tetrahedra. The A cations in P21/m amphiboles occur at the x ¼ z position, and the electron density at the A site shows a single peak. In C2/m amphiboles, the A cations may distribute themselves between two special positions (x ½ z and 0 y 0) which both repeat themselves within the A cavity. Therefore, the electron density at the A site is built up by 2 to 4 coalescing peaks; C2/m fluoro-amphiboles generally have Na strongly ordered at the x ½ z position, and show two more or less linked peaks. When approaching the transition, we found clear evidences of an incipient loss of the 21 local symmetry. This process can be followed monitoring the changes in shape of the difference Fourier peak, which in turn can be used together with the increasing similarity of the two independent O5-O6-O5 angles as a structural indicator of the approaching transition. Because the results of X-ray diffraction analysis are averaged over the whole crystal volume, both these features may also indicate that the transition proceeds rather continuously at slightly different T in different crystal domains. Cámara, F., Oberti, R., Iezzi, G., and Della Ventura, G. (2003) The P21/m<->C2/m phase transition in synthetic amphibole Na NaMg Mg5 Si8 O22 (OH)2: thermodynamic and crystal-chemical evaluation. Physics and Chemistry of Minerals, 30, 570 581.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
