A Hexagonal Perovskite Intergrowth Compound: La2Ca2MnO7

Abstract

Perovskite and related compounds have attracted considerable interest during the last two decades because of their extraordinary physical properties. High-T C cuprate superconductors and colossal magnetoresistance (CMR) manganese oxides are just two examples of a number of interesting materials with such perovskite-related structures. [1±4] Although diverse compositions and structure types have been identified for perovskite-related compounds, they primarily involve alternate stacking of the perovskite layers with layers of other structural types. The Ruddlesden ± Popper phases (ABO 3 ) n AO [5,6] are a well-known family of perovskite intergrowth compounds; in these compounds, multiple perovskite layers n octahedra thick alternate with single AO rock salt layers. The perovskite-related AO-rich intergrowth compounds (AO) n ABO 3 have also been obtained in cuprate systems such as Bi 2 Sr 2 CuO 6 and Tl 2 Sr 2 CuO 6 . [1,5] In the known perovskite-related intergrowth compounds, tetragonal perovskite layers (that is, the (001) plane of cubic perovskite) are most frequently observed, and hexagonal perovskite layers (that is, the (111) plane of cubic perovskite) are rare. This structural preference clearly originates from the synergistic relationship between the interfacial structures adopted by the perovskite layers and other intergrowth components. For example, both cubic perovskite and rock salt structures have identical (001) planes with the composition ªAOº; therefore, the topology of the interfacial structure remains unchanged or undergoes only a small distortion when they form an intergrowth compound. In contrast, the surface of the (111) perovskite block is a close packed AO 3 layer, and a hexagonal intergrowth compound can form only when the insert layer fits into this particular structural arrangement with favorable energy over the perovskite structure. One example of what can be regarded as hexagonal perovskite intergrowth compounds is the BaNiO 3 type and its derivatives. BaNiO 3 is a hexagonal modification of the perovskite structure in which AO 3 forms a hexagonal close packing and the NiO 6 octahedra share two opposite faces to form one-dimensional chains along the c axis. The intergrowth of cubic perovskite and the BaNiO 3 type with variable periodicities of the cubic and hexagonal layer results in various polytype structures. [5,7] However, these compounds are generally considered to belong to the perovskite family rather than being new intergrowth compounds. Owing to the distinctive contribution of perovskite layers to the physical properties, such as superconductivity and colossal magnetoresistance, it is of interest to search for hexagonal perovskite intergrowth compounds and to see how the physical properties are influenced by different kinds of perovskite layers.During studies on the Ca/La/Mn/O system, we identified the new compound La 2 Ca 2 MnO 7 . The empirical formula of this phase was established initially by a systematic synthetic study in the phase diagram. Chemical analysis indicated that...

Synthesis and crystal structure of Ba2La2MnW2O12

“…One example is the hexagonal perovskite intergrowth family of A n+1 B n O 3n+3 (A 2 O) consisting of alternative stacking of perovskite layers and [Ca 2 O] sheet. Several complex manganese oxides were found in this family, for instance, La 2 Ca 2 MnO 7 (n = 1) [1], La 2 Ba 0.8 Sr 0.6 Ca 1.6 Mn 2 O 10 (n = 2) [2] and La 4 Ba 2.6 Ca 1.4 (Mn 4 Ca)O 19 (n = 5) [3]. Another interesting perovskite-related family is the series A n B n−δ O 3n that is based also on the similar structural principle but the [Ca 2 O] layers are vacant [4][5][6].…”

Synthesis and Structure ofn= 5 Member of the An+1MnnO3n+3(A2O) Series

“…The graphite-like Ca 2 O sheet is an essential structural unit in the A n +1 Mn n O 3 n +3 (Ca 2 O) series. In the other members, such as La 2 Ca 2 MnO 7 ( n = 1), La 2 Ba 0.8 Sr 0.6 Ca 1.6 Mn 2 O 10 , and Ba 5 Ru 2 O 10 ( n = 2), the Ca 2 O sheets exhibit a regular graphite-like symmetry, and the oxygen atoms exhibit a 6-fold distortion according to the higher symmetry of the structures. In the La 4 Ba 2.6 Ca 1.4 (Mn 4 Ca)O 19 , however, the graphite-like Ca 2 O sheet is distorted due to the low symmetry.…”

“…One example is the (A 3 A‘BO 6 ) α (ABO 3 ) 3 β phase in which different one-dimensional chain structures are formed by stacking octahedra and trigonal prisms in ordered sequences . Recently, several new compounds belonging to a hexagonal perovskite intergrowth family of A n +1 B n O 3 n +3 (A‘ 2 O) were identified in manganate and ruthenate systems. − Their structures are built up by alternative stacking of the hexagonal perovskite blocks and the graphite-like [Ca 2 O] sheets. For example, the n = 1 members, Ln 2 Ca 2 MnO 7 (Ln = La, Nd, Sm), , comprise single hexagonal perovskite blocks [Ln 2 MnO 6 ] and graphite-like sheets [Ca 2 O], and likewise the n = 2 members, La 2 Ba 0.8 Sr 0.6 Ca 1.6 Mn 2 O 10 7 and Ba 5 Ru 2 O 10 , comprise 2H-hexagonal perovskite blocks and graphite like sheets.…”

“…Recently, several new compounds belonging to a hexagonal perovskite intergrowth family of A n +1 B n O 3 n +3 (A‘ 2 O) were identified in manganate and ruthenate systems. − Their structures are built up by alternative stacking of the hexagonal perovskite blocks and the graphite-like [Ca 2 O] sheets. For example, the n = 1 members, Ln 2 Ca 2 MnO 7 (Ln = La, Nd, Sm), , comprise single hexagonal perovskite blocks [Ln 2 MnO 6 ] and graphite-like sheets [Ca 2 O], and likewise the n = 2 members, La 2 Ba 0.8 Sr 0.6 Ca 1.6 Mn 2 O 10 7 and Ba 5 Ru 2 O 10 , comprise 2H-hexagonal perovskite blocks and graphite like sheets. In this paper, we report the synthesis and crystal structure of a new compound, La 4 Ba 2.6 Ca 1.4 (Mn 4 Ca)O 19 , which can be considered as the n = 5 member in the A n +1 Mn n O 3 n +3 (Ca 2 O) family with one-fifth of the Mn replaced by Ca.…”