The superconductivity of gallium in porous glasses with various pore sizes and in opals is studied using a superconducting quantum interference device magnetometer. The single and double superconducting phase transitions are observed for different samples. Magnetization hysteresis loops are also measured and found to be dependent on pore sizes and geometry. The changes in magnetization below about 6.4 K are treated within the framework of models for granular superconductors, while the alterations in magnetization near 7.1 K are treated as a result of the superconducting phase transition in a coexistent structural modification of confined gallium. X-ray diffraction measurements are performed to confirm the presence of such an additional gallium modification. The results obtained allow us to suggest that studies of magnetization at low temperatures can be used to get information about the geometry of the pore network and distribution of Josephson links in porous composite materials.

The melting and freezing phase transitions of mercury in a porous glass were studied by NMR and acoustic techniques. The NMR measurements provided direct information on the total amount of liquid mercury versus temperature. A depression of the phase transition temperatures and pronounced hysteresis between melting and freezing were found. Acoustic measurements showed that the freezing process was irreversible while the melting process consisted of reversible and irreversible temperature ranges. The use of longitudinal and transverse acoustic waves made it possible to obtain information about the origin of reversible and irreversible behavior upon melting. In particular, we found that the complete melting of confined mercury can be acoustically detected using only longitudinal and not transverse waves. The broadening of melting is explained by the formation of a liquid layer on the mercury solid surface, and freezing was driven by the pore geometry with no visible precursor effects.

Results of NMR studies of ${}^{23}\mathrm{Na}$ in $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ confined within molecular sieves MCM-41 with pore size 37 and 20Å and SBA-15 with pore size 52Å are presented. ${}^{23}\mathrm{Na}$ spin-lattice relaxation and line shape were measured in a large temperature range up to 535 K covering the bulk ferroelectric phase transition point. It is shown that confined $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ below the bulk sodium nitrite melting point consists of two parts with relaxation times which differ by two orders in magnitude. A portion of $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ exhibits bulk-like properties with the ferroelectric phase transition in the vicinity of the bulk transition temperature. The bulk-like $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ prevails below and near the ferroelectric phase transition and its amount decreases strongly when temperature approaches the bulk melting point. Fast nuclear relaxation in another portion of confined $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ revealed very high molecular mobility. This portion increases with increasing temperature and dominates above 510 K. It was suggested that fast relaxation corresponds to the melted or premelted state of confined $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ caused by confinement. Temperature evolution of the ${}^{23}\mathrm{Na}$ NMR line confirms such a suggestion. The amount of $\mathrm{Na}\mathrm{N}{\mathrm{O}}_2$ which possesses high molecular mobility depends on pore size and is maximal for the MCM-41 porous matrix with 20Å pore size. The correlation time of electric field gradient fluctuations was found for this part to be similar to those in viscous liquids with the activation energy of about 0.42 eV.

We have performed nuclear magnetic resonance (NMR) measurements on gallium confined in opal. In a cooling process, between 260 and $220\mathrm{K}$, the NMR spectra of gallium in opal exhibit two separate peaks at $\sim 122.57948$ (low frequency, LF) and $\sim 122.58189\mathrm{MHz}$ (high frequency, HF). These two peaks can be interpreted as the existence of two liquid gallium phases in opal. These two liquid phases of gallium have different freezing points with a transition temperature near $260\mathrm{K}$. The intensity of the LF signal decreases with the increase in intensity of the HF signals. The rapid changes of the Knight shift are at $\sim 240\mathrm{K}$ for the LF signal and at $\sim 183\mathrm{K}$ for the HF signal. The relaxation time $T_1$ of the NMR signals for ${\mathrm{Ga}}^{71}$ rapidly changes at $\sim 240\mathrm{K}$ for the LF signal and at $\sim 183\mathrm{K}$ for the HF signal, which suggests that these two kinds of liquid gallium have different freezing temperatures, $\sim 240$ and $\sim 183\mathrm{K}$ for the HF and LF phase, respectively.

The influence of the pore network geometry and interparticle long-distance electric coupling on the ferroelectric phase transition in small particles embedded into mesoporous matrices is considered. It was shown that the temperature of the ferroelectric phase transition in a system of electrically linked particles can be well different from that in isolated small particles. In particular, the model suggests an explanation for the weakening of size-effects on the ferroelectric phase transition in confined geometry.

We have studied the behavior of gallium in porous glass. Except for a few lines of α-Ga, the main pattern of the x ray does not fit in with any previously reported phase of gallium. By resistivity measurements, just above a superconducting transition, an anomalous peak at 6.3 K was observed. Lack of x-ray patterns of β-Ga and δ-Ga suggests that the 6.4 K transition might be due to a new phase of gallium or a β phase in strong disorder. © 1996 The American Physical Society.