Isovalent Substitution Research Articles

Due to the potential applications for spintronics devices, diluted ferromagnetic semiconductors (DMS) have received extensive attention for decades. However, in classical Ⅲ–Ⅴ based DMS material, such as (Ga,Mn)As, heterovalent (Ga<sup>3+</sup>, Mn<sup>2+</sup>) doping results in lack of individual control of carrier and spin doping, and seriously limited chemical solubility. The two difficulties prevent furtherincrease of the Curie temperature of the Ⅲ–Ⅴ based DMS. To overcome these difficulties, a series of new types of DMS with independent spin and charge doping have been synthesized, such as Ⅰ–Ⅱ–Ⅴ based LiZnAs and Ⅱ–Ⅱ–Ⅴ based (Ba,K)(Zn,Mn)<sub>2</sub>As<sub>2</sub>. In these new materials, isovalent (Zn,Mn) substitution is only spin doping, while charge is independently doped by heterovalentsubstitution of non-magnetic elements. As a result (Ba,K)(Zn,Mn)<sub>2</sub>As<sub>2</sub> obtains the reliable record of Curie temperature (230 K) among DMS in which ferromagnetic ordering is mediated by itinerate carriers. In this review, we summarize the recent development of the new DMS materials with following aspects: 1) the discovery and synthesis of several typical new DMS materials; 2) physical properties studies with muon spin relaxation and in-situ high pressure techniques; 3) single crystal growth, Andreev reflection junction based on single crystal and measurements of spin polarization.

Crystalline samples of the (La1 – yEuy)0.7Pb0.3MnO3 (y = 0, 0.2, 0.4, 0.5, 0.6, and 0.8) manganite solid solutions have been grown. Temperature dependences of the specific heat, thermal expansion, and high-intensity magnetocaloric effect have been investigated. A comparative analysis of the effect of isovalent cationic substitution on the thermal and caloric parameters of the ferromagnetic phase transition has been made. It has been shown that the growth of the Eu atom concentration leads to a decrease in the phase transition entropy and an increase in the baric coefficient dT/dp. Field and temperature dependences of the magnetocaloric effect have been established. It is demonstrated that, varying the ratio of cations, one can obtain the solid solutions with the maximum magnetocaloric effect value in fields of up to 6 kOe in a temperature range of 90–340 K. Owing to the similar reduced relative cooling powers, the investigated solid solutions can be used as reference solid-state cooling media in designing the cascade cooling systems.

(La1-xPrx)2Mo2O9 powders were synthesized by solid state reaction for x = 0.00, 0.10, 0.25, 0.50, 0.75 and 1.00. Dense pellets were obtained by conventional sintering in air. Their thermal stability and thermal conductivity were measured from 100 to 700 °C and their cell parameters refined from X-ray powder diffraction pattern by the Rietveld method. A 50 mol% isovalent substitution of lanthanum in La2Mo2O9 by praseodymium stabilizes the high temperature β phase while reducing the thermal conductivity of the parent compound by 11-18%. For a praseodymium content x higher than 0.75, the thermal conductivity increases and a phase transition similar to that of La2Mo2O9 is observed except that the room temperature phase appears to be this time triclinic in symmetry.

Optical spectra of cadmium sulfide heterolayers obtained by the method of isovalent substitution on cubic zinc sulfide single-crystal substrates are analyzed. The experimental differential reflectance spectra R'ω of substrates and heterolayers each contain two peaks, which are characteristic of direct-gap semiconductors of the cubic modification. The main peak A on the curves R'ω is consistent with the band gap Eg β-ZnS and β-CdS, and the peak B is shifted from it to high energies by the magnitude of the spin-orbit interaction Δso, the value of which for β-CdS was 0.33 eV. A small shift (10-30 meV) of the maximum transmission spectrum of the CdS/ZnS heterostructure to the low-energy side with respect to Eg β-CdS is adequately explained by the Bouguer law.

Abstract The formation of antisite defects in Ge_20Te_80 and Ge_15As_4Te_81 vitreous alloys in the form of tin atoms in tellurium sites and tellurium atoms in germanium sites is shown by emission Mössbauer spectroscopy with the ^119 mm Sn(^119 m Sn), ^119 m Te(^119 m Sn), ^125Sn(^125Te), and ^125 m Te(^125Te) isotopes. It is shown that the isovalent substitution of germanium atoms by tin atoms does not vary the symmetry of the local surrounding of germanium sites, while tin and tellurium atoms reconstruct their local surrounding in sites unnatural for them.

Effect of isovalent substitution of lanthanum atoms on the slab structure of indates SrLa1–xNdxInO4

Through first-principles calculations, we report on the phonon-limited transport properties of two-dimensional (2D) hexagonal MSe (M = Ge, Sn, and Pb) compounds, which can be seen as a new family of 2D group-IV selenides established by the isovalent substitutions of germanium and tellurium in layered ${\mathrm{Ge}}_{4}{\mathrm{Se}}_{3}\mathrm{Te}$ phase [Angew. Chem. Inter. Edit. 56 10204 (2017)]. We find that 2D PbSe exhibits low values of sound velocity (800--2030 m/s), large $\mathrm{Gr}\stackrel{\ifmmode \ddot{}\else \"{}\fi{}}{\mathrm{u}}\mathrm{neisen}$ parameters $(\ensuremath{\sim}1.93)$, low-lying optical modes $(\ensuremath{\sim}20.02\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$, and strong optical-acoustic phonon coupling. These intrinsic properties mainly stem from strong phonon anharmonicity, which greatly suppress the phonon transport and therefore give rise to an ultralow thermal conductivity $(\ensuremath{\sim}0.26\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ for 2D PbSe at room temperature. Our studies may offer perspectives for applications of thermoelectricity and motivate further experimental efforts to synthesize MSe compounds.

The possibility of obtaining zinc selenide and zinc sulfide layers of hexagonal modification by isovalent substitution method is shown. They are characterized by intensive luminescence which is formed by the dominant annihilation of bound excitons for α-ZnSe and recombination on donor-acceptor pairs for α-ZnS. The resulting radiation covers the violet wavelength range. Quantum radiation efficiency reaches η = 10–12% for α-ZnSe and η = 5–8% for α-ZnS. The radiation is characterized by high temperature stability and repeatability of characteristics and parameters.

Isovalent substitution of S by Se in LaOBiS2−xSex has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS2 structural instability, we have studied the local structure of LaOBiS2−xSex () using temperature dependent Bi L3-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS2 sub-lattice is largely distorted in LaOBiS2 (x = 0.0), with two in-plane Bi–S1 distances separated by ∼0.4 Å instead LaOBiSSe (x = 1.0) showing much smaller local disorder with two in-plane Bi–Se distances in the plane being separated by ∼0.2 Å. Temperature dependent study shows that the two Bi–S1 distances are characterized by different bond strength in LaOBiS2 (x = 0.0) while it is similar for the Bi–Se distances in LaOBiSSe (x = 1.0). The out of plane Bi–S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS2 layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS2−xSex is an important factor to describe differing electronic and thermal transport of the two compounds.

A correlated material in the vicinity of an insulator-metal transition (IMT) exhibits rich phenomenology and a variety of interesting phases. A common avenue to induce IMTs in Mott insulators is doping, which inevitably leads to disorder. While disorder is well known to create electronic inhomogeneity, recent theoretical studies have indicated that it may play an unexpected and much more profound role in controlling the properties of Mott systems. Theory predicts that disorder might play a role in driving a Mott insulator across an IMT, with the emergent metallic state hosting a power-law suppression of the density of states (with exponent close to 1; V-shaped gap) centered at the Fermi energy. Such V-shaped gaps have been observed in Mott systems, but their origins are as-yet unknown. To investigate this, we use scanning tunneling microscopy and spectroscopy to study isovalent Ru substitutions in Sr3(Ir1-xRux)2O7 (0 ≤ x ≤ 0.5) which drive the system into an antiferromagnetic, metallic state. Our experiments reveal that many core features of the IMT, such as power-law density of states, pinning of the Fermi energy with increasing disorder, and persistence of antiferromagnetism, can be understood as universal features of a disordered Mott system near an IMT and suggest that V-shaped gaps may be an inevitable consequence of disorder in doped Mott insulators.

We report thermoelectric properties of Ir$_{1-x}$Rh$_x$Te$_2$ ($0 \leqslant x \leqslant 0.3$) alloy series where superconductivity at low temperatures emerges as the high-temperature structural transition ($T_s$) is suppressed. The isovalent ionic substitution of Rh into Ir has different effects on physical properties when compared to the anionic substitution of Se into Te, in which the structural transition is more stable with Se substitution. Rh substitution results in a slight reduction of lattice parameters and in an increase of number of carriers per unit cell. Weak-coupled BCS superconductivity in Ir$_{0.8}$Rh$_{0.2}$Te$_2$ that emerges at low temperature ($T_c^{zero}$ = 2.45 K) is most likely driven by electron-phonon coupling rather than dimer fluctuations mediated pairing.

Effect of isovalent substitution of La3+ in Ca-doped LaNbO4 on the thermal and electrical properties

The cubic pyrochlore $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ develops a metalliclike state below the antiferromagnetic transition at ${T}_{\mathrm{N}}\ensuremath{\approx}90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, in contrast to the sharp transition to an insulating state below ${T}_{\mathrm{N}}$ of the $5d$-analog ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$. We have synthesized polycrystalline $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ under high pressure, and investigated the responses of its electronic behavior to hydrostatic pressure and isovalent substitutions of ${\mathrm{Ca}}^{2+}$ for ${\mathrm{Cd}}^{2+}$. Three characteristic anomalies are identified in the electrical transport and magnetic properties of $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$, signaling an intimate correlation between charge and spin degrees of freedom. Interestingly, we found that the metalliclike state of $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ below ${T}_{\mathrm{N}}$ is very fragile and can be suppressed by a small hydrostatic pressure of $\ensuremath{\le}1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ or substitution of 5--10% ${\mathrm{Ca}}^{2+}$ for ${\mathrm{Cd}}^{2+}$, resulting in a resistivity behavior similar to that of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$. In addition, the resultant insulating state below ${T}_{\mathrm{N}}$ is very robust against pressure, and the resistivity evolves gradually into two distinct activated regions under higher pressures. We constructed a temperature-pressure phase diagram for $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ and discussed its peculiar metalliclike state in terms of the electronic itinerancy/localization dichotomy via side-by-side comparisons with the related compounds ${A}_{2}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}(A=\mathrm{Ca},\mathrm{Hg})$ and ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$. Our results demonstrate that these cubic $4d/5d$ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.

Abstract Perovskite‐structured (ABO3) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare‐earth nickelates (RNiO3) thin films is investigated with controlled A‐site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three‐electrode system in O2‐saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A‐site element ionic radius which lowers the conductivity of RNiO3 (R = La, La0.5Nd0.5, La0.2Nd0.8, Nd, Nd0.5Sm0.5, Sm, and Gd) films, with LaNiO3 being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La0.2Nd0.8NiO3. Moreover, the OER activity remains comparable within error through Sm‐doped NdNiO3. Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni3+ to Ni2+ as a result of oxygen vacancies, which increases the average occupancy of the eg antibonding orbital to more than one. The work highlights the importance of tuning A‐site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts.

Cu2ZnSnS4-based solar cells, which constitute an inexpensive, beyond-Si photovoltaic technology, often suffer from low open-circuit voltage and efficiency. This drawback is often attributed to disorder in the Cu–Zn sublattice of the kesterite structure. While previous experiments have reported improved performances with isovalent substitution of Cd and Ag for Cu and Zn, respectively, the fundamental driving force for such improvements remains unclear. Here, we use density functional theory to study bulk stability, defect, and surface energetics, as well as the electronic structure of these dopants in Cu2ZnSnS4. We find that Cd and Ag can increase efficiencies, depending on the dopant concentration and Cu content used during synthesis. Most importantly, we find that a low level of Cd doping can suppress disorder in the kesterite phase across all Cu concentrations, while a low level of Ag doping can do so only when Zn- and Sn-rich conditions are employed. A higher Ag content is beneficial as it stabilizes t...

$\mathrm{C}{\mathrm{a}}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ is an experimentally confirmed hybrid improper ferroelectric material, in which the electric polarization is induced by a combination of the coherent $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral rotation and tilting. In this work, we investigate the tuning of ferroelectricity of $\mathrm{C}{\mathrm{a}}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ using isovalent substitutions on Ca sites. Due to the size mismatch, larger/smaller alkaline earths prefer ${A}^{\ensuremath{'}}/A$ sites, respectively, allowing the possibility for site-selective substitutions. Without extra carriers, such site-selected isovalent substitutions can significantly tune the $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral rotation and tilting, and thus change the structure and polarization. Using the first-principles calculations, our study reveals that three substituted cases (Sr, Mg, and Sr+Mg) show divergent physical behaviors. In particular, ${(\mathrm{CaTi}{\mathrm{O}}_{3})}_{2}\mathrm{SrO}$ becomes nonpolar, which can reasonably explain the suppression of polarization upon Sr substitution observed in experiment. In contrast, the polarization in ${(\mathrm{MgTi}{\mathrm{O}}_{3})}_{2}\mathrm{CaO}$ is almost doubled upon substitutions, while the estimated coercivity for ferroelectric switching does not change. The ${(\mathrm{MgTi}{\mathrm{O}}_{3})}_{2}\mathrm{SrO}$ remains polar but its structural space group changes, with moderate increased polarization and possible different ferroelectric switching paths. Our study reveals the subtle ferroelectricity in the ${A}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ family and suggests one more practical route to tune hybrid improper ferroelectricity, in addition to the strain effect.

The optical properties have been studied using the dynamical mean-field theory on a disordered Hubbard model. Despite the fact that disorder turns a metal to an insulator in high dimensional correlated materials, we notice that it can enhance certain metallic behavior as if a chemical pressure is applied to the system resulting in an increase of the effective lattice bandwidth (BW). We study optical properties in such a scenario and compare results with experiments where the BW is changed through isovalent chemical substitution (keeping electron filling unaltered) and obtain remarkable similarities vindicating our claim. We also make the point that these similarities differ from some other forms of BW tuned optical effects.

This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric $\mathrm{B}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$. Polycrystalline samples of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of $0\ensuremath{\le}x\ensuremath{\le}0.1$. The crystal structure changes from monoclinic to tetragonal with increasing La-substitution rate $x$ at room temperature. This structural variation stems from the change in orientation of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra that form one-dimensional chains when they are in the ordered configuration, thus suggesting that lone-pair electrons play an important role in sustaining one-dimensional chains of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra. Synchronizing with the disordering of $\mathrm{Si}{\mathrm{O}}_{4}$ chains, ferroelectric phase transition temperature of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ sharply decreases as $x$ increases, and ferroelectricity finally vanishes at around $x=0.03$. The present results demonstrate that lone-pair electrons of Bi play an important role in the ferroelectricity of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ through propping the ordered structure of one-dimensional $\mathrm{Si}{\mathrm{O}}_{4}$ chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ with $x\ensuremath{\le}0.01$, where the ordered one-dimensional $\mathrm{Si}{\mathrm{O}}_{4}$ chains remain.

Isovalent substitution of Zr4+ for smaller Ti4+ was performed in the 8-layer twinned hexagonal perovskite (referred to as 8H) tantalate Ba8Ti3Ta4O24, which stabilizes a 10-layer twinned hexagonal perovskite (referred to as 10H). The formation of the 10H phase occurs at low substitution concentration ( x = 0.1) in Ba8Zr xTi3- xTa4O24 at 1300 °C and reverts back to the 8H phase upon heating at elevated temperatures. Such a 10H-to-8H phase transformation is suppressed at higher Zr-substitution contents ( x > 0.1). The approach combining simulated annealing and Rietveld refinement with compositional constrain indicates that the 10H Ba8Zr0.4Ti2.6Ta4O24 ( x = 0.4) composition adopts a simply P63/ mmc disordered structure with Zr cations preferably located in corner-sharing octahedral (CSO) sites compared to face-sharing octahedral (FSO) sites. This 8H-10H phase competition, dependent on the substitution of Zr4+ for Ti4+ and firing temperature, is discussed in terms of the FSO B-B repulsion controlled by the cationic size, as well as the stacking periodicity which affects the thermodynamic stability. Both 8H- and 10H-phase pellets of Ba8Zr xTi3- xTa4O24 exhibit comparable and poorer microwave dielectric properties than the parent 8H Ba8Ti3Ta4O24, which is characterized by cationic disorder and FSO B-B repulsion. The 8H and 10H Ba8Zr xTi3- xTa4O24 ceramics display electrical insulator behavior but with electrically heterogeneous microstructure on the bulk grains. This study demonstrates the opportunity to control the stacking periodicity for the twinned hexagonal perovskites via tuning the B-cationic size and the firing temperature.

Вплив ізовалентного заміщення атомів лантану в BaLa₄Ti₄O₁₅ на будову його шаруватої перовськітоподібної структури