Strongly frustrated magnetism of the metallic pyrochlore oxide ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ has been revealed by single crystal study. While Pr $4f$ moments have an antiferromagnetic RKKY interaction energy scale of $|T^{*}|=20\mathrm{K}$ mediated by Ir $5d$-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a $\ln T$ dependence in the resistivity, emerges and leads to a partial screening of the moments below $|T^{*}|$. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.

A novel synthesis of the title compound, C₂S₃N₃ (1) is reported. X- and K-band EPR spectra on dilute solutions of 1 indicate delocalisation of the unpaired spin density over both heterocyclic rings in agreement with DFT calculations. An XRPD study indicates that it crystallises in two morphologies with both phases formed during vacuum sublimation. The XRPD studies indicate that on cooling below 230 K, only the triclinic phase (P) becomes detectable, whereas on warming above 320 K, just the monoclinic phase (P2₁/c) becomes observed. The crystal structure of the monoclinic phase has been examined by variable temperature single crystal X-ray diffraction in the region 300–225 K and reveals a regular π-stacked structure. A crystal structure of the triclinic phase is reported at 150 K and exhibits a dimeric π-stacked motif. Susceptibility measurements show that the monoclinic phase is paramagnetic whereas the triclinic phase is diamagnetic. This radical exhibits thermal hysteresis with a wide range of bistability; EPR and magnetic susceptibility measurements indicate Tc_↓ = 234 K, and Tc_↑ = 317 K. The magnetic behaviour of the monoclinic phase is consistent with strong antiferromagnetic exchange interactions between open shell doublet states (J = −320 K) along the π-stacking direction, although significant inter-stack interactions are required to model the data adequately. In contrast the dimeric phase is essentially diamagnetic, with the residual paramagnetism indicating a very large singlet–triplet separation (|2J| > 2000 K). The magnetic exchange interactions in both phases are probed through a series of DFT calculations using the broken-symmetry approach. These confirm the presence of strong magnetic exchange interactions along the π-stacking direction in the high temperature phase (2J = −182 K), but with additional interstack interactions which are an order of magnitude smaller. Calculations on the triclinic phase indicate that it is best considered as a dimer with an open-shell singlet state with a very large singlet–triplet separation (2J = −2657 K). The magnitude of J for both phases from theory and experiment are in good agreement. The origin of the thermal hysteresis is attributed to the presence of two energetically similar structures which have a low energy barrier to interconversion. The thermodynamic parameters associated with the interconversion process have been probed by DSC studies. It confirms the first order nature of the transition with Tc_↓ = 232.3 K (ΔH_↓ = 1.41 kJ mol⁻¹, ΔS_↓ = 6.0 J mol⁻¹ K⁻¹) and Tc_↑ = 320.5 K (ΔH_↑ = 1.86 kJ mol⁻¹, ΔS_↑ = 5.8 J mol⁻¹ K⁻¹).

We measured zirconium tungstate's elastic constants $C_{ij}$. This compound shows relatively soft, nearly isotropic elastic constants with normal Poisson ratios and no approach to Born instability. ${\mathrm{Z}\mathrm{r}\mathrm{W}}_2{\mathrm{O}}_8$ shows normal ambient-temperature elastic constants $C_{ij}$, but remarkable $d{C}_{ij}/dT$ that show dominant low-frequency acoustic-vibration modes. From the bulk modulus, we estimated the total ambient-temperature thermodynamic Grüneisen parameter as $\gamma =-1.2$. The $dB/dT$ slope gives a Grüneisen parameter $\gamma =-7$. The 300–0 K bulk-modulus increase ($40\%$) seems unprecedented and breaks Birch's law of corresponding states.

Spin fluctuations are reported near the magnetic field—driven quantum critical point in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$. On cooling, ferromagnetic fluctuations evolve into incommensurate correlations located at ${\mathbf{q}}_{\mathbf{0}}=\pm \left(\delta ,\delta \right)$, with $\delta =0.14\pm 0.04\mathrm{r}.\mathrm{l}.\mathrm{u}.$ At low temperatures, an in-plane magnetic field induces a sharp intradoublet resonant excitation at an energy $E_0=g{\mu }_B{\mu }_{0}H$ with $g=3.8\pm 0.2$. The intensity is localized at the zone center, indicating precession of spin density extending $\xi =6\pm 2\AA$ beyond the $4f$ site.

Single-crystal x-ray-diffraction measurements have been carried out on ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$near and below the phase transition it exhibits at $T_{C^{\prime }}\sim 195\mathrm{K}.$ ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$ was recently discovered as the first, and to date only, superconductor that displays the cubic pyrochlore structure at room temperature. Superlattice Bragg peaks show an apparently continuous structural transition at $T_{C^{\prime }},$ however, their behavior is unconventional. The evolution with temperature of these Bragg intensities show anomalously strong temperature dependence at low temperatures, where it falls off as $I_0\left(1\mathrm{}-{\mathrm{BT}}^2\right),$ and resolution limited critical-like scattering is seen above $T_{C^{\prime }}.$ High-resolution measurements show that the high-temperature cubic Bragg peaks split on entering the low-temperature phase, indicating a (likely tetragonal) lowering of symmetry below $T_{C^{\prime }}.$

The elastic tensor of monocrystalline ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ was determined near $300\mathrm{K}$ as a function of pressure, using pulse-echo ultrasound in a large-volume moissanite anvil cell. An unusual decrease in bulk modulus with increased pressure was observed. A framework-solid-based nonlinear model with many degrees of freedom predicts the observed behavior. We also observe that ${\mathrm{ReO}}_3$, a similar framework solid but lacking the necessary degrees of freedom, fails to display softening. Additionally, the pressure-induced phase transition from $\alpha \text{−}{\mathrm{ZrW}}_2{\mathrm{O}}_8$ (cubic) to $\gamma \text{−}{\mathrm{ZrW}}_2{\mathrm{O}}_8$ (orthorhombic) is found to take place at $\approx 0.5\mathrm{GPa}$, a result confirmed by Raman spectroscopy.