Covalency effects on the magnetism of EuRh2P2

In experiments, the ternary Eu pnictide EuRh2P2 shows an unusual coexistence of a non-integral Eu valence of about 2.2 and a rather high Neel temperature of 50 K. In this paper, we present a model which explains the non-integral Eu valence via covalent bonding of the Eu 4f-orbitals to P2 molecular orbitals. In contrast to intermediate valence models where the hybridization with delocalized conduction band electrons is known to suppress magnetic ordering temperatures to at most a few Kelvin, covalent hybridization to the localized P2 orbitals avoids this suppression. Using perturbation theory we calculate the valence, the high temperature susceptibility, the Eu single-ion anisotropy and the superexchange couplings of nearest and next-nearest neighbouring Eu ions. The model predicts a tetragonal anisotropy of the Curie constants. We suggest an experimental investigation of this anisotropy using single crystals. From experimental values of the valence and the two Curie constants, the three free parameters of our model can be determined.Comment: 9 pages, 5 figures, submitted to J. Phys.: Condens. Matte

Tuning the valence in ternary Eu-pnictides: the series EuPd_1-xAg_xP and EuPd_1-xAu_xAs

The series of ternary Eu-pnictides EuPd1-xAgxP and EuPd1-xAuxAs were synthesized by heating the elements and characterized by means of X-ray powder diffraction. The unusual behaviour of the lattice constants are explained by geometric and electronic factors, in particular the change of the Eu valence. In EuPdP and EuPdAs, the Eu atoms adopt a temperature-dependent mixed valent state which tends towards divalency while substituting Pd by Ag or Au. This is investigated by means of temperature-dependent X-ray diffraction, TB-LMTO-ASA band structure calculations and resonant photoemission (PE) experiments using synchrotron radiation. The calculated density of states (DOS) was compared with the results of the PE experiments. The measured partial DOS, as well as the LMTO-DOS, of EuPdP is marked by a high DOS at the Fermi level. A van Hove singularity in the band structure leads to a logarithmic DOS-peak. If the Fermi level coincides with this peak, valence instabilities are expected. The results of the LMTO band calculations are in good agreement with the experimental facts, leading to the conclusion that the range of valence instabilities of europium is limited to x<0.4 in EuPd1-xAgxP and x<0.15 in EuPd1-xAuxAs

Pressure-driven valence change in ternary Eu pnictides

We observed a structural phase transition with extremely anisotropic changes of the lattice parameters as a function of pressure at 2.6 GPa in EuPdP, which crystallizes in the hexagonal layered structure type. On the basis of the results of pressure-dependent x-ray diffraction experiments on the isostructural series APdP and APdAs (A = Sr or a trivalent rare-earth element) we show that the phase transition in EuPdP is accompanied by a valence change of the Eu. Strong but continuous changes of the lattice parameters with increasing pressure, which are due to increase of the Eu valence, were observed in EuNiP, EuPtP and EuPdAs, too. An estimation of the average Eu valence in these compounds leads to preferred values of the order of 2 n/6

First-order phase transitions in EuCo2P2\mathrm{EuCo_2P_2} and SrNi2P2\mathrm{SrNi_2P_2}

First-order phase transitions with strong and extremely anisotropic changes of the lattice parameters were observed in the ThCr2Si2 structure-type compounds EuCo2P2 and SrNi2P2. At room temperature, with increasing pressure the phase transition occurs in SrNi2P2 at 4 kbar and in EuCo2P2 at 30 kbar which is in the latter probably accompanied by a valence change of Eu. On the basis of single-crystal data of ACo2P2 (A = Ca, Sr, La, Ce, Pr, Nd, Eu) at ambient pressure and temperature we discuss the pressure dependence of the bond lengths in these compounds

First- and second-order phase transitions in ternary europium phosphides with ThCr2Si2\mathrm{ThCr_2Si_2}-type structure

The ThCr2Si2- type compounds EuFe2P2 and EuRu2P2 exhibit as a function of pressure continuous phase transitions which are accompanied by extremely strong changes of the lattice parameters and particularly of the P–P distance dP−P along the tetragonal c-axis. In the isostructural integralvalent LaT2P2 compounds similar phase transitions of second (T=Fe) and first order (T=Co) occur as a function of pressure indicating that the T element essentially determines the nature of the phase transition. The different types of phase transitions are explained phenomenologically

X-ray absorption fine structure of rare earth doped (La,Sr)2CuO4\mathrm{(La,Sr)_2CuO_4}

The local structure around the Cu atoms in rare earth (RE) doped (La,Sr)2CuO4 has been studied by means of measuring the X-ray absorption fine structure (XAFS) at the Cu K-edge. On the local scale the tilting angle of the oxygen octahedra surrounding a Cu atom depends on the Sr and RE content. No anomalies indicating charge modulation in this system have been found

First-order phase transitions in the ThCr2Si2\mathrm{ThCr_2Si_2}-type phosphides ARh2P2\mathrm{ARh_2P_2} (A = Sr, Eu)

We observed for the first time a first-order phase transition with strong and extremely anisotropic changes of the lattice parameters in compounds crystallizing in the ThCr2Si2 structure type. The phase transition occurs SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On the basis of single-crystal data of ARh2P2 (A = Ca, Sr, Ba, Eu) at ambient pressure and temperature we discuss the PP distance in the framework of band-structure calculations at the first-order phase transition the PP state changes from a ‘no-bond’ to a ‘single-bond’ state

Looking for crossmodal correspondences between classical music and fine wine

Background Wine writers sometimes compare wines to pieces of music, a particular musical style or artist, or even to specific musical parameters. To date, though, it is unclear whether such comparisons merely reflect the idiosyncratic matches of the writers concerned or whether instead they reflect more general crossmodal matching tendencies that would also be shared by others (e.g., social drinkers). In our first experiment, we looked for any consensual patterns of crossmodal matching across a group of 24 participants who were presented with four distinctive wines to taste. In our second experiment, three of the wines were presented with and without music and 26 participants were asked to rate the perceived sweetness, acidity, alcohol level, fruitiness, tannin level, and their own enjoyment of the wines. Results The results of experiment 1 revealed the existence of a significant agreement amongst the participants in terms of specific classical music - fine wine pairings that appeared to go particularly well (or badly) together. For example, Tchaikovsky’s String Quartet No 1 in D major turned out to be a very good match for the Château Margaux 2004 (red wine). Meanwhile, Mozart’s Flute Quartet in D major, K285 was found to be a good match for the Pouilly Fumé (white wine). The results of experiment 2 revealed that participants perceived the wine as tasting sweeter and enjoyed the experience more while listening to the matching music than while tasting the wine in silence. Conclusions Taken together, the results of the two experiments reported here suggest that people (social drinkers) share a number of crossmodal associations when it comes to pairing wines and music. Furthermore, listening to the appropriate classical music can enhance the overall experience associated with drinking wine. As such, our findings provide prima facie evidence to support the claim that comparing a wine to a particular style of music (as documented in the work of a number of wine writers) might provide the social drinker with useful clues about the sensory properties that they should expect to perceive in a wine should they eventually get to taste it

The anomalous valence state of Eu and magnetic order in EuRh2P2\mathrm{EuRh_2P_2}

According to x-ray absorption, Mössbauer effect measurements, lattice constants and static susceptibility, Eu in the novel compound EuRh$_2$P$_2$ is non-integral valent with a valence of the order of 2.2. On the other hand, the compound orders antiferromagnetically below 50 K, the $^{151}$Eu Mössbauer isomer shift is rather independent of temperature, and the resistivity shows no anomalies in the paramagnetic region. Hence, according to existing theories the Eu 4f level cannot be strongly hybridized with the conduction band as in usual intermediate-valent compounds. We therefore propose a considerable participation of the 4f electrons in a covalent Eu - P bond. However, the absence of the ESR signal of EuRh$_2$P$_2$ as well as the small but finite temperature dependence of the L$_{111}$ valence clearly demonstrate that the hybridization of the 4f electrons with the conduction band is not negligible in this compound