The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R $($) over bar$$ 3 or C-3i(2) consists of puckered six-membered silicate rings (Si6O18) linked by CU2+ ions and H2O groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si4+ and CU2+ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H2O motions with those of the ring is found for the Raman-active modes below 430 cm(-1). The normal modes and frequencies of the hypothetical isolated ring with C-3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H2O. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.

Several members of a new family of (Hg,Tl) (Ba,Sr) Can−2Cun−2Ox high-temperature superconductors have been synthesized at elevated pressures and temperatures. These compounds, which are analogs to the HgBaCa and TlBaCa layered cuprates, are multi-phased and have superconducting-transition temperatures which exceed 100 K. Incorporation of Hg appears to stabilize several of the Tl compounds, including a double-layer Tl/Sr system in a manner similar to the role that Pb plays in the Tl/Sr and Bi/Sr systems.

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R $$\bar 3$$ or C3i2) consists of puckered six-membered silicate rings (Si6O18) linked by Cu2+ ions and H2O groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si4+ and Cu2+ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H2O motions with those of the ring is found for the Raman-active modes below 430 cm-1. The normal modes and frequencies of the hypothetical isolated ring with C3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H2O. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.

The results of studies of new Hg based cuprate superconductors are reported. Several members of a new family of (Hg,Tl)(Ba,Sr)Can−2CunOx high temperature superconductors have been synthesized. These compounds, which are analogs to the Hg-Ba-Ca- and Tl-Ba-Ca-layered cuprates, are multi-phased and have superconducting transition temperatures above 100 K. Incorporation of Hg appears to stabilize several of the Tl-compounds, including a double layer. Tl/Sr system, in a manner similar to the role that Pb plays in the Tl/Sr- and Bi/Sr-systems. It has been suggested that recent reports of resistive Tc's above 200 K in Hg based samples are due to the presence of free Hg. Magnetization measurements of such a sample confirm this hypothesis.

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R\(\bar 3\) or C 3i 2 ) consists of puckered six-membered silicate rings (Si6O18) linked by Cu2+ ions and H2O groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si4+ and Cu2+ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H2O motions with those of the ring is found for the Raman-active modes below 430 cm-1. The normal modes and frequencies of the hypothetical isolated ring with C3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H2O. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.

The normal modes of vibration and their frequencies are calculated for beryl, a mineral whose crystal structures (space group D6h2) consists of six-membered silicate rings (Si6O18) linked by Be2+ and Al3+ ions. A valence force potential is used, consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si4+, Be2+, and Al3+ ions. The force constants are determined by fitting the calculated frequencies to the results of a complete study of the Raman spectra of a large single crystal. The calculated frequencies are in reasonable agreement with experiment, permitting unambiguous assignment of normal modes to the observed spectral lines. In several cases, the resulting interpretation of spectral features differs significantly from those published previously. Considerable mixing of Al and Be motions with those of the ring is found for the Raman-active modes near 750 cm−1 and above 850 cm−1, respectively. The normal modes and frequencies of the hypothetical isolated ring with C6h symmetry are determined by neglecting all interactions between the rings and the surrounding Be and Al atoms. This identification of normal modes characteristic of six-membered silicate rings and the effect of the environment of these modes may prove useful in the interpretation of infrared and Raman spectra of amorphous silicates.

The homologous series HgBa2Can-1CunO2n+2+delta (n = 1, 2,3,4) are the newest and most promising of the layered high-T(c) superconducting materials, as demonstrated in setting the highest T(c) among the known superconducting materials. The first of its series, HgBa2CuO4+delta, contains only one Cu-O plane per unit cell and is superconducting with T(c) about 94 K. The superconducting transition temperature as a function of pressure, T(c)(P), was measured to 4.5 GPa. T(c) was determined from ac susceptibility measurements and pressure was determined near T(c) from shifts in ruby fluorescence peaks. T(c) increases monotonically with dT(c)(P)/dP almost-equal-to 1.8 K/GPa at ambient pressure. The volume as a function of pressure, V(P), was measured to 10 GPa. The lattice parameters were determined by energy-dispersive x-ray-diffraction methods and pressure was determined from shifts in ruby fluorescence peaks. The volume decreases monotonically producing an isothermal bulk modulus B(V) = 104 +/- 17 GPa at ambient pressure. Both results were combined to produce T(c)(V), which is more readily compared to theoretical models.

The homologous series ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+2+\mathrm{\ensuremath{\delta}}}$ (n=1,2,3,4) are the newest and most promising of the layered high-${\mathit{T}}_{\mathit{c}}$ superconducting materials, as demonstrated in setting the highest ${\mathit{T}}_{\mathit{c}}$ among the known superconducting materials. The first of its series, ${\mathrm{HgBa}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$, contains only one Cu-O plane per unit cell and is superconducting with ${\mathit{T}}_{\mathit{c}}$ about 94 K. The superconducting transition temperature as a function of pressure, ${\mathit{T}}_{\mathit{c}}$(P), was measured to 4.5 GPa. ${\mathit{T}}_{\mathit{c}}$ was determined from ac susceptibility measurements and pressure was determined near ${\mathit{T}}_{\mathit{c}}$ from shifts in ruby fluorescence peaks. ${\mathit{T}}_{\mathit{c}}$ increases monotonically with ${\mathit{dT}}_{\mathit{c}}$(P)/dP\ensuremath{\approxeq}1.8 K/GPa at ambient pressure. The volume as a function of pressure, V(P), was measured to 10 GPa. The lattice parameters were determined by energy-dispersive x-ray-diffraction methods and pressure was determined from shifts in ruby fluorescence peaks. The volume decreases monotonically producing an isothermal bulk modulus ${\mathit{B}}_{\mathit{V}}$=104\ifmmode\pm\else\textpm\fi{}17 GPa at ambient pressure. Both results were combined to produce ${\mathit{T}}_{\mathit{c}}$(V), which is more readily compared to theoretical models.

Substantial improvements have been made in the sensitivity of an ac susceptibility measurement system which determines the pressure dependence of the superconducting transition temperature, Tc(P). The pressure, P, is determined near Tc from the position of the R1 and R2 fluorescence peaks from ruby chips. The system was used to determine Tc(P) for several high Tc single crystals, but was not sufficiently sensitive to determine Tc(P) of polycrystalline samples for HgBa2CuO4+δ. This motivated us to improve the sensitivity of our system. Other parts of the system and the data analysis procedures also have been improved. As a demonstration, Tc(P) of HgBa2CuO4+δ polycrystal to 4 GPa is presented. The improvement of the system using the diamond anvil cell opens the possibility of measuring Tc above 10 GPa and on smaller samples.

Pressure is an important research tool in understanding the nature of superconductivity and in the development of new superconducting materials. Experiments have been performed at NRL at pressures to 7 GPa on single crystals of Tl2Ba2CaCu2O8+δ, Tl2Ba2Ca2Cu3O10−γ, and NdBa2Cu3O7−δ. All three materials exhibit a positive dTc/dP which decreases with increasing pressure; in several instances, an extremium in Tc(P) is found. For Tl2Ba2Ca2Cu3O10−x, Tc was observed to increase from 116 K at atmospheric pressure, to a value as high as 131.8±0.5 K at 7 GPa. Measurements of Tc(P) on NdBa2Cu3O7−δ were found to be in conflict with an empirical prediction for this material.

The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group D3h2) consists of three-membered silicate rings (Si3O9) linked by Ba and Ti4+ ions. Factor-group analysis dictates that certain normal modes involve the motion of only the ring atoms. On the assumption that mode mixings and splittings due to inter-ring interactions are small, the normal frequencies of the isolated ring of C3h symmetry are determined by fitting to suitable averages of selected frequencies in the Raman spectra. A valence force potential consisting of only central interactions between nearest neighbors and bond-bending interactions centered at the silicon atoms is used. This potential is then extended to the full crystal structure by including interactions involving the Ba 2+ and Ti4+ ions. The frequencies obtained are in excellent agreement with the infrared and Raman spectra, requiring only minor adjustment of the force constants obtained for the isolated ring. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.

The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is measured as a function of pressure to 6 GPa. The pressure applied to these materials is directly measured near ${\mathit{T}}_{\mathit{c}}$, which is determined by monitoring the ac susceptibility as a function of temperature. It has been suggested that the apparent absence of superconductivity in ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is due, at least in part, to the interaction of 4f electrons of ${\mathrm{Pr}}^{3+}$ with the Cu-O plane. In light of this speculation, applying pressure to ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ might be expected to increase the interaction of 4f electrons of ${\mathrm{Nd}}^{3+}$ with electrons in the Cu-O plane and reduce ${\mathit{T}}_{\mathit{c}}$. Our results indicate that ${\mathit{T}}_{\mathit{c}}$ increases with applied pressure.

The superconducting transition temperature T(c) for NdBa2Cu3O7-delta is measured as a function of pressure to 6 GPa. The pressure applied to these materials is directly measured near T(c), which is determined by monitoring the ac susceptibility as a function of temperature. It has been suggested that the apparent absence of superconductivity in PrBa2Cu3O7-delta is due, at least in part, to the interaction of 4f electrons of Pr3+ with the Cu-O plane. In light of this speculation, applying pressure to NdBa2Cu3O7-delta might be expected to increase the interaction of 4f electrons of Nd3+ with electrons in the Cu-O plane and reduce T(c). Our results indicate that T(c) increases with applied pressure.

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R3 or C~i ) consists of puckered six-membered silicate rings (Si6Ols) linked by Cu 2§ ions and HaO groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si 4+ and Cu 2+ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H20 motions with those of the ring is found for the Raman-active modes below 430 cm-~. The normal modes and frequencies of the hypothetical isolated ring with C3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H20. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.