Competitive Effects of Substitutions in the La_1−xSr_xMn_0.9(Mg_yGe_z)_0.1O_{3 + γ} System

Abstract

Structural, magnetic, and electrical characteristics of La_1 − xSr_xMn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ (0.15 < x < 0.30, γ = 0, γ > 0) and La_0.91Sr_0.09Mn_0.90Mg_0.10O₃, La_0.71Sr_0.29Mn_0.90Ge_0.10O₃ manganites are investigated and compared. A comparison of the properties of manganites with identical total concentrations of substituents for manganese and equivalent concentrations of Mn⁴⁺ and Mn³⁺ ions reveals several competing effects caused by Sr²⁺(4p⁶), Mg²⁺(2p⁶), Ge⁴⁺(3d¹⁰) ions and oxygen.

INTRODUCTION

The unique properties of perovskite-like manganites are due to features of the compositions’ elements, the interaction of cations with oxygen ions and among themselves, the content of oxygen, and inhomogeneities of composition and structure [1–4].

Manganese-substituting ions affect the thermodynamics of the interaction between manganites and the surrounding gas environment, and thus the concentration of oxygen. According to data in [5, 6], Mg, Zn, and Ge raise the content of superstoichiometric oxygen. At the same time, raising the concentration of strontium in La–Sr manganite reduces the concentration of oxygen [7].

The formation of local atomic, electronic, and magnetic structures of manganites when manganese is replaced with doping cations depends not only on their charge, magnetic moment, ionic radius, but on electronic configuration as well. This applies in particular to diamagnetic ions Zn²⁺(3d¹⁰) and Mg²⁺(2p⁶) with almost identical radii (0.74 and 0.72 Å [8]), which have different effects on the magnetic states and properties of manganites [9].

It is known that the double exchange interaction between manganese ions of different valence states is the most important mechanism of the formation of magnetic and transport properties of manganites [2, 10]. By destroying or modifying the bonds between Mn³⁺, Mn⁴⁺ ions and changing their concentrations, substitution cations and nonstoichiometry defects strongly alter the electromagnetic parameters of manganites. Divalent and quadrivalent ions act as acceptors and donors, but as was emphasized in [10], the number of charge carriers in manganites is not always equal to that of introduced inovalent ions, since an impurity atom is electroactive only if it is isolated from other impurity atoms.

From the viewpoint of charge compensation, replacing manganese with a combination of two- and quadrivalent ions taken in equal quantities is equivalent to introducing a doubled number of trivalent ions, but the spatial distribution of cations in the crystal lattice is of course completely different [6].

We should note the specific role of germanium in the formation of the properties of substituted manganites [6], which is due to two factors: the equality of Ge⁴⁺ and Mn⁴⁺ ions radii [8] and the possible formation of Ge–Ge pairs [11].

Like the interaction between manganese and oxygen [10, 12], the possible hybridization of d-levels of manganese and p-levels of magnesium cannot be excluded. Mg²⁺ ions can in this case participate in the transfer of charges in manganites [13].

The aim of this work was to investigate the effect of the content of strontium and oxygen on the structural, magnetic, and electrical characteristics of manganites with manganese replaced by a combination of divalent (Mg²⁺) and quadrivalent (Ge⁴⁺) ions in La_1 − xSr_xMn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ, system, and to consider the La_0.81Sr_0.19Mn_0.90${{\left( {{\text{Mg}}_{{0.5}}^{{2 + }}{\text{Ge}}_{{0.5}}^{{4 + }}} \right)}_{{0.10}}}$O₃ composition of this system as a solid solution of the corresponding components. Since different-valence manganese ions are crucial to the formation of ferromagnetism and conductivity of manganites, this work compares properties of compositions that have the same concentration of Mn⁴⁺ and Mn³⁺ ions with the substitution of $\left( {{\text{Mg}}_{{0.5}}^{{2 + }}{\text{Ge}}_{{0.5}}^{{4 + }}} \right)$ and single substitutions of Mg²⁺ and Ge⁴⁺ for manganese: ${\text{L}}{{{\text{a}}}_{{0.81}}}{\text{S}}{{{\text{r}}}_{{0.19}}}{\text{Mn}}_{{0.19}}^{{4 + }}{\text{Mn}}_{{0.71}}^{{3 + }}{{\left( {{\text{Mg}}_{{0.5}}^{{2 + }}{\text{Ge}}_{{0.5}}^{{4 + }}} \right)}_{{0.10}}}{{{\text{O}}}_{3}},$ ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.91}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.09}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Mg}}_{{{\text{0}}{\text{.10}}}}^{{{\text{2 + }}}}{{{\text{O}}}_{{\text{3}}}}{\text{,}}$ ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.71}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.29}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.10}}}}^{{{\text{4 + }}}}{{{\text{O}}}_{{\text{3}}}}{\text{.}}$

Properties of ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.81}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.19}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{{\left( {{\text{Zn}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)}_{{{\text{0}}{\text{.10}}}}}{{{\text{O}}}_{{\text{3}}}}{\text{.}}$ manganite are also presented for purposes of comparison.

The content of Mn⁴⁺ (0.19 f.u.) is slightly higher than the points of metal–semiconductor and orthorhombic–rhombohedral transitions (0.17–0.175 f.u.) in basic system ${\text{La}}_{{1 - x}}^{{3 + }}{\text{Sr}}_{x}^{{2 + }}{\text{Mn}}_{x}^{{4 + }}{\text{Mn}}_{{1 - x}}^{{3 + }}{\text{O}}_{3}^{{2 - }}$ [14], where the properties of manganites are quite sensitive to composition.

The effects of using substituents are of interest because they have different electron configurations: Mg²⁺ ions have full p-electron shells (2p⁶), while Ge⁴⁺ and Zn²⁺ have full d-shells (3d¹⁰).

EXPERIMENTAL

Polycrystalline samples were synthesized via conventional ceramic processing. The procedures and conditions of synthesis were described in [5, 6]. Final sintering was done in air over 10 h at 1473 K with subsequent cooling of the samples in the furnace. Sintered manganites were then exposed to heat treatment for 96 h at 1223 K and partial pressures of oxygen in the gas phase of ${{{\text{P}}}_{{{{{\text{O}}}_{{\text{2}}}}}}}$ = 10⁻¹ Pa and 10⁵ Pa, ensuring the production of manganites with stoichiometric oxygen content (γ = 0) and with γ > 0, respectively.

Phase composition and unit cell parameters at room temperature were determined via powder X-ray diffraction in CuK_α radiation using a Shimadzu XRD‑7000 diffractometer.

The values of non-stoichiometry index (γ) were calculated from data on the unit cell volume of stoichiometric and oxygen annealed samples, according to the algorithm proposed in [6, 15].

Specific magnetization (σ) was measured ballistically in a magnetic field of 5.6 kOe at a temperature of 80 K. The Curie point (Т_C) on the temperature dependence of magnetic permeability (µ(Т)) was determined as the temperature corresponding to the maximum of |dµ/dT|. The resistance of manganites was measured on samples in the form of tablets 4 mm thick, on the opposite planes of which copper electrodes were deposited via thermal sputtering in vacuum.

RESULTS AND DISCUSSION

All synthesized manganites had rhombohedral crystal structure. Unit cell parameters and the non-stoichiometry index of La_1 − xSr_xMn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ (0.15 < x < 0.30) manganites are presented in Table 1.

Table 1. Unit cell volume (V), lattice parameters ratio с/а and non-stoichiometry index of manganites: (I) stoichiometric samples (γ = 0); (II) samples annealed in oxygen

Unit cell volume fell as the content of strontium and oxygen rose, due to the concentration of Mn⁴⁺ ions (ion radius r(Mn⁴⁺) = 0.53 Å) rising at the expense of the concentration of Mn³⁺ (r(Mn³⁺) = 0.645 Å) as a result of the charge compensation of Sr²⁺ and O²⁻ ions.

The content of superstoichiometric oxygen and thus the concentration of cation vacancies in manganites annealed in oxygen fell as the amount of Sr rose.

According to the data shown in Fig. 1, the specific magnetization and Curie points of these manganites rose as a function of the content of strontium, and the samples annealed in oxygen had higher values of magnetic parameters than the stoichiometric specimens. These effects were due to an increased number of the pairs of manganese ions bound by double exchange interaction.

Fig. 1.

Dependences of specific magnetization (1, 1 ') and Curie point (2, 2 ') of La_1 − xSr_xMn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ manganites on the concentration of Sr: (1, 2) stoichiometric samples (γ = 0); (1 ', 2 ') samples annealed in oxygen.

Raising the с/а ratio (Table 1) weakens the antiferromagnetic interaction while strengthening the ferromagnetic interaction [16], which also contributes to an increase in magnetic parameters.

Table 2 compares properties of ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.81}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.19}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{{\left( {{\text{Mg}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)}_{{{\text{0}}{\text{.10}}}}}{{{\text{O}}}_{{\text{3}}}}$ manganite to analogous parameters of ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.91}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.09}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Mg}}_{{{\text{0}}{\text{.10}}}}^{{{\text{2 + }}}}{{{\text{O}}}_{{\text{3}}}}{\text{,}}$ ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.71}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.29}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.10}}}}^{{{\text{4 + }}}}{{{\text{O}}}_{{\text{3}}}}$, and ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.81}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.19}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{{\left( {{\text{Zn}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)}_{{{\text{0}}{\text{.10}}}}}{{{\text{O}}}_{{\text{3}}}}$ manganites with identical total concentrations of substituents for Mn and equivalent concentrations of Mn⁴⁺ and Mn³⁺ ions.

Table 2. Parameters of manganites with double and mono-substitution for manganese

The magnetization and Curie point of sample 3 are much higher than those of sample 2 mainly because of two factors: (a) the radius of the Mg²⁺ ion is much larger than that of the Ge⁴⁺ ion, which is electrically and geometrically indistinguishable from Mn⁴⁺; (b) Mg²⁺ localizes near Mn⁴ ions due to Coulomb interaction, thereby preventing its participation in double exchange interaction.

The composition of sample 1 can be presented as a solid solution of compositions 2 and 3: ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.81}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.19}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{{\left( {{\text{Mg}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)}_{{{\text{0}}{\text{.10}}}}}{{{\text{O}}}_{{\text{3}}}}$ = ${1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}\left( {{\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.91}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.09}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Mg}}_{{{\text{0}}{\text{.10}}}}^{{{\text{2 + }}}}{{{\text{O}}}_{{\text{3}}}}} \right)$ + ${1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}\left( {{\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.71}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.29}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.10}}}}^{{{\text{4 + }}}}{{{\text{O}}}_{{\text{3}}}}} \right){\text{,}}$ and the values of σ and T_C calculated according to the additivity rule ((σ)_add = 68.9 emu g⁻¹, (T_C)_add = 217 K) are close to experimental ones.

Although ionic radius of Zn²⁺ is greater than that of Mg²⁺, zinc-containing manganite 4 has higher magnetic parameters than magnesium-containing manganite (no. 1) of analogous composition, due possibly to the difference between the electron shell configurations of these ions.

Manganites La_1 − xSr_xMn_0.9(Mg_0.5Ge_0.5)_0.1O_3 + γ at 0.15 ≤ x ≤ 0.19 have semiconductor-type conductance in the 120 to 290 K range of temperatures (Fig. 2).

Fig. 2.

Temperature dependences of the resistance of La_1 − xSr_xMn_0.9(Mg_0.5Ge_0.5)_0.1O_3 + γ manganites: (1, 1 ') x = 0.15; (2, 2') x = 0.17; (3, 3 ') x = 0.19 (1, 2, 3) stoichiometric samples (γ = 0); (1 ', 2 ', 3 ') samples annealed in oxygen.

To find the energy of activation of conductivity (E_act), the dependences of resistance on temperature in the 130–220 K range were constructed in coordinates 10³ × T ⁻¹–ln R and approximated by straight lines (Fig. 3).

Fig. 3.

Approximation of experimental data on the temperature dependences of resistance by straight lines in coordinates 10³ × T ⁻¹–ln R of La_1 − xSr_xMn_0.9(Mg_0.5Ge_0.5)_0.1O_3 + γ manganites: (1, 1') x = 0.15; (2, 2 ') x = 0.17; (3, 3 ') x = 0.19; (1, 2, 3) stoichiometric samples (γ = 0); (1 ', 2 ', 3 ') samples annealed in oxygen).

Obtained values of E_act are given in Table 3. The energy of activation and the maximum resistivity of stoichiometric and oxygen-annealed samples (Table 3) fell as the content of strontium rose, and manganites with superstoichiometric oxygen contents had significantly lower E_act and ρ_max values.

Table 3. Energy of activation of conductivity and maximum resistivity (ρ_max, at T = 123 K) of stoichiometric samples (I) and ones annealed in oxygen (II)

Manganites La_0.70Sr_0.30Mn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ had metal–semiconductor transitions (Fig. 4) at temperature T_ms = 186 K in the stoichiometric sample and at 179 K in the specimen with γ = 0.01. The drop in the temperature of transition was obviously due to the existence of cation vacancies.

Fig. 4.

Temperature dependences of the resistance of manganites La_0.70Sr_0.30Mn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ with the metal–semiconductor phase transition: (1) γ = 0, T_ms = 186 K; (1 ') γ = 0.01, T_ms = 179 K.

It is interesting that the electrical characteristics of the studied manganites were more sensitive to the content of cations (and especially oxygen) than magnetic parameters. This can be explained by the heterogeneous distribution of ions and the influence of cation vacancies, which alters the band spectrum of manganites [7, 10]. Inhomogeneities can also serve as scattering centers for charge carriers. At the same time, the vacancy mechanism of diffusion can contribute to the formation of a more homogeneous structure as a result of the prolonged annealing of manganites in oxygen.

CONCLUSIONS

In manganites of the La_1 − xSr_xMn_0.90(Mg_0.5Ge_0.5)_0.10O_3 + γ system annealed at 1223 K and a partial pressure of oxygen in the gas phase of 10⁵ Pa, the concentration of superstoichiometric oxygen fell as the content of strontium rose. The value of oxygen nonstoichiometry index γ was less than the one in manganites of the base system La_1 − xSr_xMnO_3 + γ [7].

Magnetization and the Curie point of investigated manganites rose as a function of the Sr content and had higher values than the stoichiometric samples. However, these magnetic parameters differed slightly.

In contrast, electrical characteristics depended strongly on the cationic composition and content of oxygen. Manganites of the studied system at 0.15 ≤ x ≤ 0.19 had a semiconductor character of conductance in the 123–293 K range. The maximum resistivity and energy of activation of the samples fell considerably as the contents of strontium and superstoichiometric oxygen rose. Stoichiometric and oxygen-annealed samples with strontium content x = 0.30 displayed a metal–semiconductor transition at 186 and 179 K, respectively.

We compared the magnetic parameters of ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.81}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.19}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{{\left( {{\text{Mg}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)}_{{{\text{0}}{\text{.10}}}}}{{{\text{O}}}_{{\text{3}}}}{\text{,}}$ ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.91}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.09}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Mg}}_{{{\text{0}}{\text{.10}}}}^{{{\text{2 + }}}}{{{\text{O}}}_{{\text{3}}}}$ and ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.71}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.29}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.10}}}}^{{{\text{4 + }}}}{{{\text{O}}}_{{\text{3}}}}$ manganites with identical concentrations of substituents for Mn and equivalent concentrations of Mn⁴⁺ and Mn³⁺ ions. The magnetization and Curie point of Ge-substituted manganite were much higher than those of the Mg-substituted sample, and the values of σ and T_C calculated according to the additivity rule were close to experimental ones for manganite with double substitution of $\left( {{\text{Mg}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)$ for manganese.

Zinc-containing manganite ${\text{L}}{{{\text{a}}}_{{{\text{0}}{\text{.81}}}}}{\text{S}}{{{\text{r}}}_{{{\text{0}}{\text{.19}}}}}{\text{Mn}}_{{{\text{0}}{\text{.19}}}}^{{{\text{4 + }}}}{\text{Mn}}_{{{\text{0}}{\text{.71}}}}^{{{\text{3 + }}}}{{\left( {{\text{Zn}}_{{{\text{0}}{\text{.5}}}}^{{{\text{2 + }}}}{\text{Ge}}_{{{\text{0}}{\text{.5}}}}^{{{\text{4 + }}}}} \right)}_{{{\text{0}}{\text{.10}}}}}{{{\text{O}}}_{{\text{3}}}}$ had higher magnetic parameters than magnesium-containing manganite of analogous composition.

Dependences of properties of the studied manganites on their compositions were considered with allowance for a number of competitive effects and factors: the influence of strontium, manganese substituents, and content of oxygen on the number of (Mn³⁺and Mn⁴⁺) pairs linked by double exchange interaction; the c/a ratio; the configuration, the charge and radius of manganese substituents; the inhomogeneous distribution of cation vacancies and Mg²⁺ and Ge⁴⁺ ions; violation of exchange bonds between Mn⁴⁺ and Mn³⁺ ions due to the shielding of Mn⁴⁺ by Mg²⁺ ions and vacancies; and the role of the vacancy mechanism of diffusion.