arXiv subjects

Zoran Mazej

Publications and source records attributed to Zoran Mazej.

15 recordsLinked to original sources

arXiv2025

Evaluating covalency using RIXS spectral weights: Silver fluorides vs. cuprates

We investigate the electronic structure of AgF2, AgFBF4, AgF and Ag2O using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Ag L3 edge. XAS results were compared with density functional theory computations of the spectra, allowing an identification of main features and an assessment of the theoretical approximations. Our RIXS measurements reveal that AgF2 exhibits charge transfer excitations and dd excitations, analogous to those observed in La2CuO4. We propose to use the ratio of dd to CT spectral weight as a measure of the covalence of the compounds and provide explicit equations for the weights as a function of the scattering geometry for crystals and powders. The measurements at the metal site L3 edge and previous measurements at the ligand K edge reveal a striking similarity between the fluorides and cuprates materials, with fluorides somewhat more covalent than cuprates. These findings support the hypothesis that silver fluorides are an excellent platform to mimic the physics of cuprates, providing a promising avenue for exploring high-Tc superconductivity and exotic magnetism in quasi-two-dimensional (AgF2) and quasi-one-dimensional (AgFBF4) materials.

cond-mat.str-el↗

arXiv2024

Electron-overdoped Ag(II)F2: mixed-valence fluorides Ag(I)Ag(II)F3 and Ag(I)2Ag(II)F4

We have successfully prepared two novel mixed-valence compounds of silver, Ag(I)Ag(II)F3 and Ag(I)2Ag(II)F4. They may be considered to be long-sought strongly electron-overdoped Ag(II)F2. Their crystal structures indicate that both belong to the Class I (mixed-valence) family with frozen Ag(I) and Ag(II) valences. The measured Raman spectra are well-correlated with the theoretical ones. Density functional theory calculations reveal their smaller fundamental band gaps as compared to pristine AgF2, due to the presence of Ag(I) states in the valence band.

cond-mat.mtrl-sci↗

arXiv2024

Novel Ternary AgIICoIIIF5 Fluoride: Synthesis, Structure and Magnetic Characteristics

We present a new compound in the silver cobalt fluoride system, featuring paramagnetic silver (d9) and high-spin cobalt (d6), synthesized by solid state method in an autoclave under F2 overpressure. Based on powder X ray diffraction, we determined that AgIICoIIIF5 crystallizes in a monoclinic system with space group C2/c. The calculated fundamental band gap falls in the visible range of the electromagnetic spectrum, and the compound has the character of charge-transfer insulator. AgCoF5 is a ferrimagnet with one predominant superexchange magnetic interaction constant between mixed spin cations (Ag ... Co) of minus 62 meV (SCAN result). Magnetometric measurements conducted on a powdered sample allowed the identification of a transition at 128 K, which could indicate magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2024

Rearrangement of orbitals in KAgF3 due to Kugel-Khomskii mechanism: a Neutron diffraction and Density Functional Theory study

The crystal structure of KAgF3 was studied by powder neutron diffraction. KAgF3 exhibits at all temperatures an orthorhombic symmetry in space group Pnma that allows for several distortions with respect to the ideal cubic perovskite structure. At all temperatures there is a strong splitting of Ag-F distances parallel to the a,c planes that documents alternating occupation of holes in x2-y2 and z2-y2 orbitals. The orientation of the octahedron elongation and thereby the orbital order flips at a structural phase transition occurring around Ts=240K which is accompanied by a suppression of magnetic susceptibility. The orbital ordering is further enhanced in the low-temperature phase and the twisting of the AgF4 plaquettes forming the antiferromagnetic chains changes. DFT calculations show an enhancement of the magnetic interaction in the low temperature phase indicating that the transition and the orbital order are partially driven by the Kugel-Khomskii mechanism.

cond-mat.str-el↗

arXiv2023

A unique two-dimensional silver(II) antiferromagnet Cu[Ag(SO4)2] and perspectives for its further modifications

Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO4-related structure with P21/n symmetry. This quasi-ternary compound features [Ag(SO4)2]2- layers, while the remaining cationic sites may be occupied either completely or partially by Cu2+ cations, corresponding to the formula of (CuxAg1-x)[Ag(SO4)2], x = 0.6-1.0. CuAg(SO4)2 is antiferromagnetic with large negative Curie-Weiss temperature of -84.1 K and shows two characteristic ordering phenomena at 19 K and 40 K. Density functional theory calculations reveal that the strongest superexchange interaction is a two-dimensional antiferromagnetic coupling within [Ag(SO4)2]2- layers, with the superexchange constant J2D of -11.1 meV. This renders CuAg(SO4)2 the rare representative of layered Ag2+-based antiferromagnets. Magnetic coupling is facilitated by the strong mixing of Ag d(x2-y2) and O 2p states. Calculations show that M2+ sites in MAg(SO4)2 can be occupied with other similar cations such as Zn2+, Cd2+, Ni2+, Co2+, and Mg2+.

cond-mat.mtrl-sci↗

arXiv2022

Low temperature magnetism of KAgF3

KAgF$_3$ is a quasi one-dimensional quantum antiferromagnet hosting a series of intriguing structural and magnetic transitions. Here we use powder neutron diffraction, $\mu$SR spectroscopy, and Density Functional Theory calculations to elucidate the low temperature magnetic phases. Below $T_{N1}=29$K we find that the material orders as an A-type antiferromagnet with an ordered moment of 0.47$\mu_{\rm B}$. Both neutrons and muons provide evidence for an intermediate phase at temperatures $T_{N1}<T<T_{N2}$ with $T_{N2}\approx 66$ K from a previous magnetometry study. However, the evidence is at the limit of detection and its nature remains an open problem.

cond-mat.str-el↗

arXiv2022

New CuSO4-related high-temperature polymorph of AgIISO4

Silver(II) compounds exhibit powerful oxidizing properties and strong magnetic superexchange. AgSO4 is a rare fluorine-free salt of Ag(II) which found some application in organic chemistry. Here, we report a discovery of a new AgSO4 polymorph (\b{eta}). The distinct nature of the two polytypes of AgSO4 is established using powder x-ray diffraction, vibrational spectroscopy and theoretical calculations. The \b{eta} polymorph crystallizes in the monoclinic system (P21/n) and shows structural similarities with CuSO4. DFT calculations indicate very small differences in the energy of the two polymorphs AgSO4, but the relative stability of the \b{eta} polymorph should increase with temperature. The monoclinic distortion of the orthorhombic CuSO4 prototype originates from an unprecedented strong antiferromagnetic interaction between Ag sites along the unit cell diagonal.

cond-mat.mtrl-sci↗

arXiv2022

Strength of Correlations in a Silver Based Cuprate Analogue

AgF2 has been proposed as a cuprate analogue which requires strong correlation and marked covalence. On the other hand, fluorides are usually quite ionic and 4d transition metals tend to be less correlated than their 3d counterparts, which calls for further scrutiny. We combine valence band photoemission and Auger-Meitner spectroscopy of AgF and AgF2 together with computations in small clusters to estimate values of the Ag 4d Coulomb interaction U 4d and charge-transfer energy. Based on these values, AgF2 can be classified as a charge-transfer correlated insulator according to the Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material is a cuprate analogue from the point of view of correlations, suggesting that it should become a high-temperature superconductor if metallization is achieved by doping. We present also a computation of the Hubbard U in density functional "+U" methods and discuss its relation to the Hubbard U in spectroscopies.

cond-mat.str-el↗

arXiv2022

Charge Transfer and $dd$ excitations in AgF$_{2}$

Charge transfer (CT) insulators are the parent phase of a large group of today's unconventional high-temperature superconductors. Here we study experimentally and theoretically the interband excitations of the CT insulator silver fluoride AgF$_2$, which has been proposed as an excellent analogue of oxocuprates. Optical conductivity and resonant inelastic X-ray scattering (RIXS) on AgF$_2$ polycrystalline sample show a close similarity with that measured on undoped La$_2$CuO$_4$. While the former shows a CT gap $\sim$3.4 eV, larger than in the cuprate, $dd$ excitations are nearly at the same energy in the two materials. DFT and exact diagonalization cluster computations of the multiplet spectra show that AgF$_2$ is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that AgF$_2$ is at the verge of a charge transfer instability. The overall resemblance of our data on AgF$_2$ to those published previously on La$_2$CuO$_4$ suggests that the underlying CT insulator physics is the same, while AgF$_2$ could also benefit from a proximity to a charge density wave phase as in BaBiO$_3$. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.

cond-mat.supr-con↗

arXiv2022

Phase transitions and amorphization of M2AgF4 (M = Na, K, Rb) compounds at high pressure

We report the results of Raman spectroscopy high-pressure studies of alkali metal fluoroargentates (M2AgF4, where M=Na, K, Rb) associated with theoretical and x-ray diffraction studies for the K member of the series. Theoretical density functional calculations predict two structural phase transitions for K2AgF4: one from low pressure monoclinic P21/c (beta) phase to intermediate-pressure tetragonal I4!2d structure at 6 GPa, and another to high-pressure triclinic P1! phase at 58 GPa. However, Raman spectroscopy and X-ray diffraction data indicate that both polymorphic forms of K2AgF4 as well as two other fluoroargentate phases studied undergo amorphization at pressure as low as several GPa.

cond-mat.mtrl-sci↗

arXiv2021

Crystal structure, lattice dynamics and superexchange in MAgF3 1D antiferromagnets (M = K, Rb, Cs) and Rb3Ag2F7 Ruddlesden-Popper phase

With the use of lattice dynamics calculation within hybrid HSE06 framework we were able to understand vibrational spectra of MAgF3 M = K, Rb, Cs compounds. Comparative theoretical study uncovered lack of monotonicity in calculated optical phonons associated with Ag F stretching modes which can be explained through an interplay of Lewis acidity of MI cation and its size. We confirm the tetragonal unit cells of MAgF3 M=Rb, Cs at room temperature. We also theoretically predict an orthorhombic RbAgF3 polymorph as a ground state at low temperature. However, we were not able to detect it by the means of low temperature PXRD at 80 K nor low temperature Raman at 154 K due to a number of constraints. We also describe a novel Ruddlesden Popper phase of Rb3Ag2F7 which can be regarded as quasi 0D system, where superexchange coupling constant between the nearest AgII centres reaches an impressive value of minus 240.2 meV.

cond-mat.mtrl-sci↗

arXiv2021

NaZnF3 as a low-pressure analogue of MgSiO3

Solid-state systems whose properties at high pressure (exceeding 1 GPa) mimic those of MgSiO3 are of large importance in the study of the interior of planets. By means of Density Functional Theory (DFT) calculations we studied the high pressure properties of a MgSiO3 analogue, NaZnF3. We reproduce the phase-transition sequence previously reported for this compound (GdFeO3 -> CaIrO3 -> La2S3), and predict that it should undergo a two-step dissociation: decomposition into a equimolar mixture of Na2ZnF4 and NaZn2F5 at 25.4 GPa, followed by a breakdown into ZnF2 and NaF at 66.8 GPa. These processes are analogous to those predicted for compressed MgSiO3. Moreover, both Na2ZnF4 and NaZn2F5 are isostructural with analogous phases from the Mg-Si-O system. We also find that both these novel compounds are thermodynamically stable at ambient conditions (Na2ZnF4) or at low pressure of 19 GPa (NaZn2F5). Our study indicates that NaZnF3 could serve as a good low-pressure analogue of MgSiO3 exhibiting the same sequence of phase transitions, and pressure induced decomposition, but at pressures an order of magnitude lower.

cond-mat.mtrl-sci↗

arXiv2020

Lattice dynamics of KAgF3 perovskite, unique 1D antiferromagnet

Theoretical DFT calculations using GGA+U and HSE06 frameworks enabled vibrational mode assignment and partial (atomic) phonon DOS determination in KAgF3 perovskite, a low-dimensional magnetic fluoroargentate(II). Twelve bands in the spectra of KAgF3 were assigned to either IR active or Raman active modes, reaching very good correlation with experimental values (R2>0.997). Low-temperature Raman measurements indicate that the intriguing spin-Peierls-like phase transition at 230 K is an order-disorder transition and it does not strongly impact the vibrational structure of the material.

cond-mat.mtrl-sci↗

arXiv2020

Unexpected persistence of cis-bridged chains in compressed AuF3

Raman scattering measurements indicate that cis-bridged chains are retained in AuF3 even at a compression of 45 GPa - in contrast to meta-GGA calculations suggesting that structures with such motifs are thermodynamically unstable above 4 GPa. This metastability implies that novel gold fluorides (e.g. AuF2) might be attainable at lower pressures than previously proposed.

cond-mat.mtrl-sci↗

arXiv2018

The silver route to cuprate analogs

The parent compound of high-Tc superconducting cuprates is a unique Mott state consisting of layers of spin-1/2 ions arranged on a square lattice and with a record high antiferromagnetic coupling within the layers. Compounds with similar characteristics have long been searched for. Nickelates and iridates had been proposed as cuprate analogs but so far have not reached a satisfactory similarity. Here we use a combination of experimental and theoretical tools to show that the commercial compound AgF2 is an excellent cuprate analog with remarkably similar electronic parameters to La2CuO4 but larger buckling of planes. Two-magnon Raman scattering reveals a superexchange constant which reaches 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the AgF2 planes could have an antiferromagnetic coupling that matches or surpasses the cuprates.

cond-mat.str-el↗