X-ray studies of structural changes in melanophlogite with varying temperature

Abstract

The a and c unit cell dimensions of melanophlogite (MEP) have been determined in the temperature range −50 to 700 °C, showing a different expansion behavior for the low temperature α phase. The c axis length, 2c, which is smaller than a axis length in the α phase, shows a steep rise reaching the value of a at the tetragonal (α) cubic (β) transition temperature at about 65 °C, and then remains nearly constant until about 500 °C, after which contraction occurs. The crystal structures of α MEP (space group = P4 2 /nbc) and β MEP (space group = Pm3 n) were refined using a least squares refinement of a harmonic structure factor expression, using single crystal X ray diffraction data measured at seven temperatures from −50 to 200 ºC (four points for the α phase , and three points for the β phase). The average Si O distance decreased from 1.593 Å at −50 °C down to 1.573 Å at 63 °C (the estimated transition point). It then remained nearly constant in the β phase. The Si O bond distance corrected using a simple rigid body motion model remained nearly constant at 1.611 Å in the temperature range −50 to 200 °C, indicating that the negative temperature dependence is due to strong distortions of the probability density functions of the O atoms. The atomic mean square displacement, <u 2 >, of the O atoms increased steeply with increasing temperature up to the α β transition point. The low high (α β) transformation in MEP is driven by a mechanism involving atom disorder beginning in the low temperature phase.

Higher-order thermal motion tensor analysis and atom disorder in cubic melanophlogite

“…In addition to the characteristic structural changes referred to above, Nakagawa et al (2005) also noted that the difference Fourier syntheses showed significant negative values at the O atom sites (down to − 0.7 e/Å 3 ), except for the O 3 site, which was surrounded by positive fringes (illustrated later in Fig. 2).…”

“…On the other hand, the MSD of the Si atoms in MEP at 84 °C is 0.021 Å 2 , when averaged over 46 atoms, which remains similar to the values observed in common silica structures. Based on these characteristic MSD features, Nakagawa et al (2005) concluded that the structural transition in MEP is driven by an atom disorder mechanism.…”

“…However, the Si O bond distances of the former space group were more compatible with other common silicas and with the corresponding bond distances in the β phase at other temperatures. The correspondence of independent atoms between the two phases (Pm3 n for the β phase) is shown in Nakagawa et al (2005) described the details of the structural changes occurring in MEP with changes in temperature, focusing on the SiO 4 framework. These authors found that the α β transition is nearly second order or tricritical with a transition temperature (T c ) of about 65 °C, and that the mean square displacement (MSD) of the O atoms is highly anisotropic and very large, even at a temperature of 15 °C below the α β phase transition point.…”

“…Two phases have been known to exist above room temperature for the specimens from Mt Hamilton, California, USA: the high form (β) is cubic, with a space group of Pm3 n or P4 3n (Gies, 1983;Nakagawa et al, 2005), and the low form (α) exists as a (2 × 2 × 1) superstructure of the β form with a tetragonal space group of P4 2 /nbc (Žák, 1972;Nakagawa et al, 2001). In Nakagawa et al's (2005) study of MEP samples of Mt Hamilton, the β phase structure was refined with both of space groups Pm3 n and P4 3n, with harmonic temperature factors. However, the Si O bond distances of the former space group were more compatible with other common silicas and with the corresponding bond distances in the β phase at other temperatures.…”

“…The features of difference Fourier syntheses suggests a possible failure of the harmonic model to simulate such strongly distorted atomic PDFs. We carried out a least squares refinement involving higher order thermal motion tensor coefficients on space group Pm3 n using F o data and unit cell dimensions of a = 13.4104(9) Å at 84 °C (Nakagawa et al, 2005). The PDFs of the O atoms were found to be bimodal or strongly distorted from an ellipsoidal shape, and these were analyzed at the point of disorder over the domains in the α phase in both the twin and antiphase relationships.…”