Microstructure of bilayer manganite PrCa2Mn2O7 showing charge/orbital ordering

Abstract

The microstructure of the charge/orbital ordering Ruddleden-Popper phase PrCa 2 Mn 2 O 7 was studied by transmission electron microscopy along both the [001] and the [110] orientation. Three coexisting charge/orbital ordering phases CO1, CO2, and CO3 were observed along the [001] orientation at room temperature. Different from the one-dimensional modulation in the CO1 and CO2 phase, the CO3 phase is characterized by two sets of mutually perpendicular structural modulations. From [110] high angle annular dark field-scanning transmission electron microscopy, we found that the Pr atoms locate in-between the bilayer MnO 6 octahedra, which is different from the previous reports. The strongly correlated electronic systems of perovskite manganites have received great attention because of their fascinating physical properties such as colossal magnetoresistance and high spin polarity, which could find potential applications in spintronics or magnetic tunnel junction. [1][2][3][4][5][6][7][8][9] The physical mechanism of these properties was ascribed to the complicated interplays between the four degrees of freedom of lattice, charge, orbital, and spin, and the consequent ordering as well as possible phase separation. 6,[10][11][12] Recently, two distinct charge/orbital ordering (CO/OO) states were found in the Ruddlesden-Popper (RP) compound Pr(Sr 1Àx Ca x ) 2 Mn 2 O 7 . 13 Pr(Sr 1Àx Ca x ) 2 Mn 2 O 7 has a layer structure (AO) (ABO 3 ) n (n ¼ 2), where two (ABO 3 ) perovskite layers are separated by a AO rock salt layer along the long axis. The two reported CO/OO phases in Pr(Sr 0.1 Ca 0.9 ) 2 Mn 2 O 7 are the high-temperature phase CO1 phase at 330 K > T > 295 K (lattice parameters a ¼ 0.5412, b ¼ 1.0921, c ¼ 1.9234 nm, and space group Pbnm) and the lower-temperature CO2 phase at T < 295 K (a ¼ 1.0812, b ¼ 0.5475, c ¼ 1.9203 nm, and space group Am2m, Ref. 13). The CO1 phase is centrosymmetric without any spontaneous electric polarization, but the CO2 phase is a noncentrosymmetric phase with spontaneous electric polarization. 13 The thermally induced rotation of orbital stripes between these two ordering phases has attracted extensive interest. [14][15][16][17][18] However, the limited information about its microstructures has obstructed a better understanding of the macro properties and physics underneath. Therefore, we carried out a transmission electron microscopy (TEM) study on PrCa 2 Mn 2 O 7 to reveal the different aspects of the microstructure. The phases and micro domains viewed along the [001] zone axis are discussed. The heavy Pr atoms were found to locate inbetween the bilayer MnO 6 octahedra.Bulk PrCa 2 Mn 2 O 7 samples were synthesized using a traveling solvent floating zone method with high oxygen background pressure. 17,18 Powdered samples, deposited on a Cu grid coated with a holey carbon film, as well as Ar-ion milled TEM samples were used for TEM investigation. A Tecnai G 2 electron microscope equipped with a Gatan cooling and heating holder operated at 200 kV was used to record electron diffractio...

Three charge-ordered phases in bilayered Pr(Sr0.1Ca0.9)2Mn2O7

“…1 and Ca (≈71%) is mainly located at the rock-salt layer denoted as Ca(Pr)2 site in the same figure. This atomic distribution is in agreement with previous electron microscopy studies [17,19]. In this structure there are also two positions for the apical oxygens, O1 and O2.…”

“…1). Further heating leads to an intensity decrease of (2, 3/2, 0) peak until it disappears at TCO1≈360 K. The coexistence of CO1 and CO2 phases has been observed previously [17,18,[35][36][37] and some authors have proposed the occurrence of an intermediate phase with two modulations or a propagation vector k'= (½, ½, 0) [17,18].…”

“…Several studies devoted to the characterization of this non-polar to polar transition have been published in the last years, including investigations on how to manipulate this transition, either by varying the temperature or the external pressure [15][16][17]. Despite all these works, there are still some uncertainties regarding the transition mechanism between these two CO phases.…”

Chemical ordering suppresses large-scale electronic phase separation in doped manganites

“…In general, the spatial locations of the chemical dopants are random, which generates a certain kind of disorder in the system. For CMR mangnanites, it has been shown that the ordering of chemical dopants, or the cation ordering has strong influence on the physical properties for both bulk11121314151617 and superlattice structures181920212223242526. However, very few work has dealt the correlation between cation ordering and the electronic phase separation (EPS)27, despite of the fact that EPS is important for both understanding the mechanism of strong electronic correlation and for tuning the physical properties.…”

Chemical pressure effects on crystal and magnetic structures of bilayer manganites PrA2Mn2O7 (A = Sr or Ca)

“…Our recent TEM study also demonstrated the complexity of the crystal structure and phase transition of this compound. 19,20 Therefore, it is necessary to reinvestigate the crystal and magnetic structures of the series Pr(Sr 1Ày Ca y ) 2 Mn 2 O 7 .…”