Anomalous chemical pressure evolution in ThFePnN (Pn for pnictogens)

The phase relation and physical properties of the ${\mathrm{YbCu}}_{5\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ system have been investigated as functions of temperature and composition x. Powder x-ray-diffraction experiments showed that ${\mathrm{YbCu}}_{5\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ crystallizes in a single phase of the cubic ${\mathrm{AuBe}}_{5}$ -type structure in the range of 0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0. Magnetic susceptibility \ensuremath{\chi}, electrical resistivity \ensuremath{\rho}, and specific heat C measurements revealed that this system belongs to a series of dense Kondo compounds, and the characteristic temperature of Kondo effect shifts to lower temperatures with decreasing x. It was also observed that the susceptibility at zero temperature, \ensuremath{\chi}(0), the ${\mathrm{T}}^{2}$ coefficient of resistivity, A, and the electronic specific heat coefficient, \ensuremath{\gamma}, are enhanced with the decrease of x (0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0). The \ensuremath{\gamma} value increases monotonically from 210 mJ/mol ${\mathrm{K}}^{2}$ for x=1.0 to 460 mJ/mol ${\mathrm{K}}^{2}$ for x=0.125 with decreasing x. This substantial increase of \ensuremath{\gamma} with decreasing x can be explained by the 'chemical pressure' effect, which is estimated from the composition dependence of the lattice parameters. Furthermore, the coefficient A depending on 'chemical pressure' is well parallel to that for x=1.0 under hydrostatic pressures. In the composition range of 0.0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.1, the system consists of two phases with the cubic ${\mathrm{AuBe}}_{5}$ -type and the hexagonal ${\mathrm{CaCu}}_{5}$ -type structures. These results mean that the ${\mathrm{YbCu}}_{5}$ exists with the cubic ${\mathrm{AuBe}}_{5}$ -type structure as well as with the hexagonal ${\mathrm{CaCu}}_{5}$ -type one, although only the hexagonal ${\mathrm{CaCu}}_{5}$ -type ${\mathrm{YbCu}}_{5}$ has been reported so far. The value of \ensuremath{\gamma} for the hypothetical cubic ${\mathrm{YbCu}}_{5}$ of single phase is expected to reach about 500 mJ/mol ${\mathrm{K}}^{2}$ from an extrapolation of the \ensuremath{\gamma} values of ${\mathrm{YbCu}}_{5\mathrm{\ensuremath{-}}\mathrm{x}}$ ${\mathrm{Ag}}_{\mathrm{x}}$ (0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0) to x=0.

In this, we report the temperature-dependent magnetization [M(T)] in two distinct magnetic fields of 0.5 T and 5 T for Ni47Mn40−x Si x In3 (x = 1, 2, and 3) alloys. Using a phenomenological model and Maxwell’s thermodynamic relation, the values of the magnetic entropy change and specific heat capacity are calculated, and their values are also compared. The maximum magnetic entropy change and specific heat capacity peak values for different magnetic fields are both steadily reduced for the samples with x = 1 to 3 samples, which is followed by an increase in relative cooling power value. In comparison to 0.5 T magnetic field, the samples investigate the highest values of magnetic entropy change (3.32, 2.81, 2.01 J kg−1 K−1) and specific heat capacity (32.37, 14, 4.32 J kg−1 K−1) with a magnetic field of 5 T. According to this finding, the sample is more responsible for the magnetic field than chemical pressure.

The dependence of structural and magnetic properties of rare-earth orthoferrites (in their Pbnm ground state) on the rare-earth ionic radius is systematically investigated from first principles. The effects of this ‘chemical pressure’ on lattice constants, Fe–O bond lengths, Fe–O–Fe bond angles and Fe–O bond length splittings are all well reproduced by these ab initio calculations. The simulations also offer novel predictions (on tiltings of FeO6 octahedra, cation antipolar displacements and weak magnetization) to be experimentally checked. In particular, the weak ferromagnetic moment of rare-earth orthoferrites is predicted to be a linear function of the rare-earth ionic radius. Finally, the effects of applying hydrostatic pressure on structural and magnetic behavior of SmFeO3 is also studied. It is found that, unlike previously assumed, hydrostatic pressure typically generates changes in physical properties that are quantitatively and even qualitatively different from those associated with the chemical pressure.

Effect of external pressure and chemical substitution on the phase transitions in MnAs

The rich phase diagrams of magnetically frustrated pyrochlores have maintained a high level of interest over the past 20 years. To experimentally explore these phase diagrams requires a means of tuning the relevant interactions. One approach to achieve this is chemical pressure, that is, varying the size of the non-magnetic cation. Here, we report on a new family of lead-based pyrochlores A$_2$Pb$_2$O$_7$ (A = Pr, Nd, Gd), which we have characterized with magnetic susceptibility and specific heat. Lead is the largest known possible B-site cation for the pyrochlore lattice. Thus, these materials significantly expand the phase space of the frustrated pyrochlores. Pr$_2$Pb$_2$O$_7$ has an absence of long-range magnetic order down to 400 mK and a spin ice-like heat capacity anomaly at 1.2 K. Thus, Pr$_2$Pb$_2$O$_7$ is a candidate for a quantum spin ice state, despite weaker exchange. Nd$_2$Pb$_2$O$_7$ transitions to a magnetically ordered state at 0.41 K. The Weiss temperature for Nd$_2$Pb$_2$O$_7$ is $\theta_{\text{CW}}$ = $-$0.06 K, indicating close competition between ferromagnetic and antiferromagnetic interactions. Gd$_2$Pb$_2$O$_7$ is a Heisenberg antiferromagnet that transitions to long-range magnetic order at 0.81 K, in spite of significant site mixing. Below its ordering transition, we find a $T^{3/2}$ heat capacity dependence in Gd$_2$Pb$_2$O$_7$, confirmation of a ground state that is distinct from other gadolinium pyrochlores. These lead-based pyrochlores provide insight into the effects of weakened exchange on highly frustrated lattices and represent further realizations of several exotic magnetic ground states which can test theoretical models.

We have used specific heat and neutron diffraction measurements on single crystals of URu$_{2-x}$Fe$_x$Si$_2$ for Fe concentrations $x$ $\leq$ 0.7 to establish that chemical substitution of Ru with Fe acts as "chemical pressure" $P_{ch}$ as previously proposed by Kanchanavatee et al. [Phys. Rev. B {\bf 84}, 245122 (2011)] based on bulk measurements on polycrystalline samples. Notably, neutron diffraction reveals a sharp increase of the uranium magnetic moment at $x=0.1$, reminiscent of the behavior at the "hidden order" (HO) to large moment antiferromagnetic (LMAFM) phase transition observed at a pressure $P_x\approx$ 0.5-0.7~GPa in URu$_2$Si$_2$. Using the unit cell volume determined from our measurements and an isothermal compressibility $\kappa_{T} = 5.2 \times 10^{-3}$ GPa$^{-1}$ for URu$_2$Si$_2$, we determine the chemical pressure $P_{ch}$ in URu$_{2-x}$Fe$_x$Si$_2$ as a function of $x$. The resulting temperature $T$-chemical pressure $P_{ch}$ phase diagram for URu$_{2-x}$Fe$_x$Si$_2$ is in agreement with the established temperature $T$-external pressure $P$ phase diagram of URu$_2$Si$_2$.

Th-doped URu 2Si 2: influence of “Kondo holes” on coexisting superconductivity and magnetism

Main observation and conclusionWe report the design, synthesis, structure, and properties of two complex layered phosphide nitrides, AkTh2Mn4P4N2 (Ak = Rb, Cs), which contain anti‐fluorite‐type [Mn2P2] bilayers separated by fluorite‐type [Th2N2] layers as a result of the intergrowth between AkMn2P2 and ThMnPN. The new compounds are featured with an intrinsic hole doping associated with the interlayer charge transfer and a built‐in chemical pressure from the [Th2N2] layers, both of which are reflected by the changes in the lattice and the atomic position of phosphorus. The measurements of magnetic susceptibility, electrical resistivity, and specific heat indicate existence of local moments as well as itinerant electrons in relation with d‐p hybridizations. The expected dominant antiferromagnetic interactions with enhanced d‐p hybridizations were demonstrated by the first‐principles calculations only when additional Coulomb repulsions are included. The density of states at the Fermi level derived from the specific‐heat analysis are 3.5 and 7.5 times of the calculated ones for Ak = Rb and Cs, respectively, suggesting strong electron correlations in the title compounds.

Constitution, physical properties and thermodynamic modeling of the Hf-Mn system

We have investigated the substitution effect of YbCo2(Zn1–xTx)20 (T = Cu, Ga, and Cd) systems by using the experiments of X-ray powder diffraction (XRPD), specific heat, magnetic susceptibility, magnetization, and electrical resistivity in order to find out a material that approaches a quantum critical point by chemical pressure. The XRPD and electrical resistivity measurements clarify that the Cu-substitution makes the lattice constants shrink and keeps the magnetic electrical resistivity high, while the Ga- and the Cd-substitution show opposite relation of the Cu-substitution. However, we could not detect clear substitution effect in the specific heat, magnetic susceptibility, and magnetization measurements of Cu-substitution system within our experiments. It is necessary that to study the Cu-substitution samples that have higher x value at lower temperature.

The heavy-fermion metal YbRh$_2$Si$_2$ realizes a field-induced quantum critical point with multiple vanishing energy scales $T_{\rm N}(B)$ and $T^\ast(B)$. We investigate their change with partial non-isoelectronic substitutions, chemical and hydrostatic pressure. Low-temperature electrical resistivity, specific heat and magnetic susceptibility of Yb(Rh$_{1-x}$T$_x$)$_2$Si$_2$ with T=Fe or Ni for $x\leq 0.1$, magnetic fields $B\leq 0.3$~T (applied perpendicular to the c-axis) and hydrostatic pressure $p\leq 1.5$~GPa are reported. The data allow to disentangle the combined influences of hydrostatic and chemical pressure, as well as non-isoelectronic substitution. In contrast to Ni- and Co-substitution, which enhance magnetic order, Fe-substitution acts oppositely. For $x=0.1$ it also completely suppresses the $T^\ast$ crossover and eliminates ferromagnetic fluctuations. The pressure, magnetic field and temperature dependences of $T^\ast$ are incompatible with its interpretation as Kondo breakdown signature.

We have studied crystal structure and physical properties of REO1−yFyBiS2 (Ce1−xNdxO1−yFyBiS2 and Nd1−zSmzO1−yFyBiS2) with three different F concentration (y = 0.7, 0.5, and 0.3) to investigate relationship between the emergence of superconductivity and crystal structure in the REO1−yFyBiS2 series. REO1−yFyBiS2 is suitable for discussing chemical pressure effect on physical properties because the RE site at the blocking layer can be fully or partially substituted by various RE3+ ions, which could systematically tune the lattice volume. With increasing chemical pressure (with decreasing lattice volume), lattice constant of a-axis systematically decreases while lattice constant of c-axis does not show a remarkable change, indicating that the RE site substitution basically affect the lattice constant of a-axis. On the other hand, lattice constant of c-axis can be tuned by F concentration. On the basis of magnetic susceptibility measurements, we have obtained three kinds of superconductivity phase diagram with y = 0.7, 0.5, and 0.3 as a function of RE concentration (chemical pressure). For all the systems with y = 0.7, 0.5, and 0.3, chemical pressure basically increases superconducting transition temperature (Tc) with increasing chemical pressure. Having compared these three phase diagrams, we have suggested that there are at least two important structure parameters, (1) lattice contraction along a-axis, (2) optimal lattice contraction ratio (c/a), are essential for the emergence of superconductivity and increase in Tc in REO1−yFyBiS2.

Abstract

The crystal structure of the quasi-one-dimensional cuprate ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ has been studied under high pressure using neutron powder diffraction methods. Full structure refinements were undertaken, using the Rietveld method, with data acquired between room pressure and $0.55(1)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature. The compressibility of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ is anisotropic, a consequence of the ordered nature of the anion vacancies in this material. The effect of high pressure and chemical substitution on the crystal structure of the ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{CuO}}_{3}$ system is discussed and it is suggested that the substitution of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{Ca}}^{2+}$ may constitute a ``chemical pressure'' effect in this solid solution. The availability of accurate bond length compressibility data for ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ may now allow an appraisal of the effect of applied pressure on the remarkable electronic properties of this material, through appropriate band structure calculations.

We report the antimony (Sb) doping effect in a prototype system of iron-based superconductors LaFeAsO1−y F y (y=0, 0.1, 0.15). X-ray powder diffraction indicates that the lattice parameters increase with Sb content within the doping limit. Rietveld structural refinements show that, with the partial substitution of Sb for As, the thickness of the Fe2As2 layers increases significantly, whereas that of the La2O2 layers shrinks simultaneously. So a negative chemical pressure is indeed “applied” to the superconducting-active Fe2As2 layers, in contrast to the effect of positive chemical pressure by the phosphorus doping. Electrical resistance and magnetic susceptibility measurements indicate that, while the Sb doping hardly influences the SDW anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of y=0.1. In the meantime, the superconducting transition temperature can be raised up to 30 K in LaFeAs1−x Sb x O1−y F y with x=0.1 and y=0.15. The Sb doping effects are discussed in term of both J 1–J 2 model and Fermi Surface (FS) nesting scenario.