First-principles modeling of the infrared spectrum of Fe- and Al-bearing lizardite

The theoretical vibrational properties of a series of Fe- and Al-bearing lizardite models have been determined at the density functional theory level. Each periodic model displays a single cationic impurity substituted at an octahedral or tetrahedral site of a supercell of lizardite (Mg3Si2O5(OH)4) containing 162 atoms. The isovalent Fe2+ for Mg2+ substitution has been considered, as well as the heterovalent substitution of Fe3+ or Al3+ for Mg2+ or Si4+. Comparison of the theoretical absorption spectra with previously reported experimental spectra of natural and laboratory-grown lizardite samples allows us to propose an interpretation for most of the observed bands. Although the identification of specific bands related to octahedral Fe2+ in FTIR spectra is challenging, broad bands at 3584 and 3566 cm−1 reflect the occurrence of octahedral Al3+ and Fe3+, respectively, in the natural samples. These broad bands likely overlap with potential contribution related to tetrahedral Al3+. It is suggested that the modification of the H-bonding pattern related to the incorporation of trivalent ions at tetrahedral sites has an overall broadening effect on the interlayer-OH stretching bands of lizardite.

Electrical conductivity in sodium chlorate crystals

The electrical conductivity, σ of single crystals of sodium chlorate (m.p. 256 °C) grown from solution is studied in the temperature range 100 to 250 °C. A plot of lg ( σT ) against 1000/ T gives a fairly continuous curve. It is resolved into three straight lines by a special graphical procedure. The experimental data can then be represented by σT = 2 x 10 -4 e -6376/ T + 8 x 10 9 e -19012/ T +10 29 e -41386/ T . The addition of the divalent cationic impurity Ba 2+ increased the conductivity between 130 and 80 °C, indicating a cation vacancy jump mechanism. The activation energy for the migration of cations is estimated to be 0.55 ± 0.02 eV and the energy of formation of thermal defects to be 2.18 eV. The conductivity changes are discussed in relation to the enhanced optical absorption coefficient in the long wavelength fundamental region and also in relation to the anomalous thermal expansion and dielectric constant of the crystal at the higher temperatures. Above 200 °C the conductivity increases at a higher rate. Several causes were considered and the effect is most probably due to products of thermal decomposition.