New high-pressure phase of HfTiO4 and ZrTiO4 ceramics

We studied the high-pressure effects on the crystalline structure of monoclinic HfTiO4 and ZrTiO4. We found that the compressibility of these ceramics is highly non-isotropic, being the b-axis the most compressible one. In addition, the a-axis is found to have a small and negative compressibility. At 2.7 GPa (10.7 GPa) we discovered the onset of an structural phase transition in HfTiO4 (ZrTiO4), coexisting the low- and high-pressure phases in a broad pressure range. The new high-pressure phase has a monoclinic structure which involves an increase in the Ti-O coordination and a collapse of the cell volume. The equation of state for the low-pressure phase is also determined.Comment: 16 pages, 5 figures, 26 references, Article in Pres

High-pressure x-ray diffraction study of SrMoO4 and pressure-induced structural changes

SrMoO4 was studied under compression up to 25 GPa by angle-dispersive x-ray diffraction. A phase transition was observed from the scheelite-structured ambient phase to a monoclinic fergusonite phase at 12.2(9) GPa with cell parameters a = 5.265(9) A, b = 11.191(9) A, c = 5.195 (5) A, and beta = 90.9, Z = 4 at 13.1 GPa. There is no significant volume collapse at the phase transition. No additional phase transitions were observed and on release of pressure the initial phase is recovered, implying that the observed structural modifications are reversible. The reported transition appeared to be a ferroelastic second-order transformation producing a structure that is a monoclinic distortion of the low-pressure phase and was previously observed in compounds isostructural to SrMoO4. A possible mechanism for the transition is proposed and its character is discussed in terms of the present data and the Landau theory. Finally, the EOS is reported and the anisotropic compressibility of the studied crystal is discussed in terms of the compression of the Sr-O and Mo-O bonds.Comment: 36 pages, 9 figures, 1 tabl

Superionicity and Polymorphism in Calcium Fluoride at High Pressure

We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF2 at high P-T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists in a decrease of the normal-> superionic critical temperature with compression. This unexpected effect can be explained in terms of a P-induced softening of a zone-boundary $X$ phonon which involves exclusively fluorine displacements. Also we find that superionic conductivity is absent in the high-P cotunnite phase. Instead, superionicity develops in a new low-symmetry high-T phase that we identify as monoclinic (space group P2_1/c). We discuss the possibility of observing these intriguing phenomena in related isomorphic materials.Comment: 5 pages, 5 figure

Tight-Binding study of the electronic and magnetic properties of an L1_0 ordered FeCu alloy

We have calculated the electronic structure of the tetragonal L1$_0$ ordered FeCu by solving self-consistently a tight-binding Hamiltonian for s, p and d electrons. We have found by total energy calculation that this structure is ferromagnetic. In addition, we have determined that the equilibrium ratio between the interlayer and the intralayer lattice parameters is 0.947.Comment: 7 pages latex, 4 postscript figures included. To appear in Phys. Lett.

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO4 under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO4 beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO4 as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms. (C) 2014 Elsevier Masson SAS. All rights reserved.Research sponsored by Spanish MINECO (MAT2010-21270-C04-01/04 and CSD2007-00045). Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.Vilaplana Cerda, RI.; Lacomba-Perales, R.; Gomis, O.; Errandonea, D.; Meng, Y. (2014). Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa. Solid State Sciences. 36:16-23. https://doi.org/10.1016/j.solidstatesciences.2014.07.003S16233

Pressure-induced alpha-to-omega transition in titanium metal: A systematic study of the effects of uniaxial stress

We investigated the effects of uniaxial stress on the pressure-induced alpha-to-omega transition in pure titanium (Ti) by means of angle dispersive x-ray diffraction in a diamond-anvil cell. Experiments under four different pressure environments reveal that: (1) the onset of the transition depends on the pressure medium used, going from 4.9 GPa (no pressure medium) to 10.5 GPa (argon pressure medium); (2) the a and w phases coexist over a rather large pressure range, which depends on the pressure medium employed; (3) the hysteresis and quenchability of the w phase is affected by differences in the sample pressure environment; and (4) a short term laser-heating of Ti lowers the alpha-to-omega transition pressure. Possible transition mechanisms are discussed in the light of the present results, which clearly demonstrated the influence of uniaxial stress in the alpha-to-omega transition.Comment: 16 pages, 6 figures, 1 tabl

Euskararen ergatibitateaz

Lan honen helburua azken 30 urteotan euskal ergatibitatearen inguruan, batez ere gramatika sortzailearen barruan, plazaratu diren ideia eta hausnarketen sintesi kritikoa egitea da. Bai euskal hiztunok eta bai gramatika deskriptibo batzuek (EGLUk 1991, A grammar of Basque-k 2003, de Rijk-ek 2008 eta Sareko Euskal Gramatikak) euskara ergatibotzat hartu izan dute, baina adituen artean ikuspegi ezberdinak daude ergatibitatearen nondik-norakoak definitzerako orduan. Alde batetik, ergatibitatea egiletasunarekin lotzen dutenak daude. Levinek (1983) euskararen NOR Aditzen Hipotesiaren bidez euskararen ergatibitatea egile rolarekin lotzen du, eta euskara hizkuntza aktibotzat hartzen. Ortiz de Urbinak (1989) hizkuntza ergatibo-absolutibotzat du euskara, eta gainerakoan, Levinen (1983) proposamenarekin bat egiten du. Baina hori bai: azken honek azaldu ezin duen subjektutasunaren arazoari azalpena topatzen dio Ortiz de Urbinak (1989). Laka (2006) ere bat dator Levinen (1983) ikuspegiarekin, baina horretaz gain, ergatibitatea berezko kasuari lotu beharrekoa dela aipatzen du. Bertzetik, ergatibitatearen ikuspegi formalaren alde egiten dutenak daude; hau da, ergatibitatea argumentu kopuruarekin edo egiturazko kasuarekin lotzen dutenak. Lakak (1993a) ergatibitatea Nahitaezko Kasuaren Parametroarekin erlazionatuta dagoela erraten du, eta ondorioz, ergatiboa agertzeko absolutiboa ere egotea ezinbertzekoa dela. Fernándezek (1997) ere Lakaren (1993a) azterbidea jarraitzen du, baina horretaz gain, aditz ezergatiboen analisi xeheago baten berri ere ematen du. Rezac, Albizu eta Etxepareren (2014) erranetan, ergatibitatea ez dago derrigorrez egiletasunari lotuta: Denbora kategoriari loturiko egiturazko konfigurazio batzuen menpe dago. Bada goiko bi ikuspegi multzoen tartekorik ere. Oyharçabalen (1992) iritziz, hizkuntza ergakusatiboa da euskara: ergatiboa berezkoa du eta absolutiboa egiturazkoa. Horretaz gain, aipatzen du absolutibo kasua osagarriari zein subjektuari lotuta egon daitekeela. Bertzalde, aldakortasun dialektala eta ergatibitatea uztarturik, Levinen (1983) Nor Aditzen Hipotesiak ez du euskararen funtzionamendu orokorra azaltzen, mendebaldeko eta erdialdeko kasu-sistematik hurbilago baitago ekialdekotik baino. Lakak (2006) ere ez ditu ekialdeko hizkerak kontuan hartzen, baina irakurlea horretaz ohartarazten du. Oyharçabalek (1992), iparraldekoa izaki, kontuan hartzen ditu iparekialdeko hizkera horiek. Horretaz gain, Berrok eta Etxeparek (2017) aditz motak eta subjektuak hizkerekiko erlazioan jartzeko sailkapen bat proposatzen dute. Aldaik (2010), bere aldetik, euskalkien arteko aldeak aurkitzeko asmoz, aditz inergatibo bakunen gaineko azterketa bat egiten du. Ergatibitatearen gaineko ikuspegiak eta aldakortasunaz errandakoak kontuan harturik, honelaxe definituko nuke euskararen ergatibitatea: batetik, euskara morfologikoki ergatiboa eta sintaktikoki akusatiboa da; bertzetik, egiletasunari lotuta agertzen zaigu mendebaldeko hizkeretan batez ere, baina salbuespenak topatzen ditugu ipar-ekialdeko hizkeretan; azkenik, ergatiboa berezkoa dela erratea ezinezkoa egiten zait horren kontrako argudioak ugari baitira; baina, aldi berean, egiturazkoa dela errateak ere arazoak sortzen dizkit, absolutiboak beti ez baitu ergatiboa agertzea baldintzatze