The crystals of Na₂Ti₃O₇ were obtained by crystallization from flux. The structure of the compound was refined from X-ray diffraction data collected on a four-circle diffractometer (2θ/θ scanning technique, λMoK _α radiation, graphite monochromator, θ_max = 40°). The crystals are monoclinic a = 9.133(2) Å, b = 3.806(1) Å, c = 8.566(2) Å, β = 101.57(3)°, sp. gr. P2₁/m, Z = 2, ρ_calcd = 3.435 g/cm³, R = 0.035, 1241 reflections with I ≥ 2σ(I). The geometric characteristics of the Ti-polyhedra are analyzed as to their positions in the trioctahedral ribbon. The polymorphism of the {Ti₃O₇}²⁻ anionic radical in the structures of Na₂Ti₃O₇ and PbTi₃O₇ is described. The topology and dimensionality of the { Ti₃O₇}²⁻ anionic radical are demonstrated to depend on the type of the large cations located at the lattice points of the hexagonal close packing characteristic of both structures.

The crystal structure of the chabazite K with the formula (K1.33Na1.02Ca0.84)[Al4Si8O24] · 12.17H2O from late hydrothermalites in the Khibiny alkaline massif (Kola Peninsula) is established by X-ray...

We studied the low-temperature specific heats of two antiferromagnets with the two-dimensional square-kagome structure, i.e., ${\mathrm{KCu}}_6{\mathrm{AlBiO}}_4{\left({\mathrm{SO}}_4\right)}_5\mathrm{Cl}$ $\left({\mathrm{KCu}}_6\right)$ and ${\mathrm{Na}}_6{\mathrm{Cu}}_7{\mathrm{BiO}}_4{\left({\mathrm{PO}}_4\right)}_4{[\mathrm{Cl},(\mathrm{OH}\left)\right]}_3$ $\left({\mathrm{NaCu}}_7\right)$, with the structural difference that there are interlayer ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{NaCu}}_7$. Both materials show no magnetic ordering down to 50 mK. At zero field, the $C/T$ of ${\mathrm{KCu}}_6$ has a finite value when the temperature is close to 0 K. Under the magnetic field, an apparent $T^2$ dependence appears and its coefficient is progressively suppressed by the field. For ${\mathrm{NaCu}}_7$, the specific heat exhibits the $T^2$ dependence at zero field and under fields. The ratio of the quadratic coefficients of ${\mathrm{KCu}}_6$ and ${\mathrm{NaCu}}_7$ at high fields is inversely proportional to that of the squared Weiss temperatures, which indicates these two compounds host the same ground state under fields. Our results suggest that the interlayer ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{NaCu}}_7$ play a negligible role in determination of its ground state. We discuss the possible quantum-spin-liquid states in these compounds and further directions to pursue based on our results.