On the Structural and Magnetic Properties of B-site Ordered Double Perovkites

Perovskites are one of the compound families that can be tailored to perform multiple scientifically and also technologically exciting and essential tasks. At the very early stage of perovskite history, right at the same time as the structure was solved, the first application possibility was found in electronics. The introduction of BaTiO3 as a dielectric material greatly enhanced the performance of ceramic capacitors of the time. Not to a surprise, it is still used in the very same application, some 80 years after the discovery. Since then, perovskites have found their way to widespread and emerging applications, from capacitors to solid oxide fuel cells, piezoelectric transducers and actuators to second harmonic generators in lasers and to spintronics, not to forget phenomena purely of academic interest. This thesis aims to bring its portion to the pool of scientific data to be used to expand the knowledge of the perovskite structure and properties within. New double perovskites with tuned crystal structures were synthesised by introducing a set of elements from the Periodic Table and by applying a variety of solid-state chemistry synthesis methods. Not only synthesis and characterisation, but also multivariate analysis and prediction of new compounds were performed. The time and effort required to design, make and characterise each perovskite may be enhanced by utilising multivariate data analysis of the existing data. The main target of the multivariate data analysis was to find new ferromagnetic B-site ordered double perovskite candidates. The proposed candidate compound should have ferromagnetic transition temperature within a reasonable temperature range in order to have an application without extensive or impractical effort to reach the operating temperature. Samples of two non-perovskite systems, (Sr,Ba)2FeSbO6 and Y2CuTiO6, were transformed into a variety of double perovskites with different crystal structures by applying synthesis techniques ranging from ambient pressure cation substitution to high-pressure high-temperature treatment. The SIMCA multivariate data analysis provided insight into ferromagnetic B-site ordered double perovskites. Agreement between the measured and predicted ferromagnetic transition tempera- tures were found as a result of the chosen multivariate analysis methods. Some non-perovskite compounds, previously synthesized but uncharacterised double perovskites and completely new stoichiometries were proposed as new ferromagnetic double perovskite candidates.Perovskiitit ovat yhdisteryhmä, jotka voidaan räätälöidä useisiin tieteen ja tekniikan kannalta kiinnostaviin ja tärkeisiin sovelluksiin. Perovskiittitutkimuksen alkuaikoina, yhdisteen rakenteen selvittämisen aikaan, niille löydettiin sovellus elektroniikassa. BaTiO3 osoittautui keraamisissa kondensaattoreissa käytettyjä eristeitä paremmaksi materiaaliksi ja yhdistettä käytetään edelleen samassa sovelluskohteessa, noin 80 vuotta keksimisensä jälkeen. Perovskiitit ovat löytäneet tiensä laajalle levinneisiin sekä kehitteillä oleviin sovelluksiin kondensaattoreista polttokennojen elektrolyytteihin, pietsosähköisistä antureista ja toimilaitteista lasereiden taajuusmuuntimiin sekä spintroniikan sovelluksiin, unohtamatta akateemisen tarkastelun kannalta mielenkiintoisia ilmiöitä. Väitöskirja tuo osansa tieteellisen tiedon tarjontaan. Uusia kaksoisperovskiitteja voidaan valmistaa valitsemalla sopivat alkuaineet ja soveltamalla kemian synteesimenetelmiä. Työssä ei ainoastaan valmistettu ja määritetty uusien kaksoisperovskiittien ominaisuuksia, vaan käytettiin myös monimuuttuja-analyysiä. Uusien yhdisteiden suunnitteluun, valmistamiseen ja ominaisuuksien määrittämiseen käytettyä aikaa ja työpanosta voidaan tehostaa soveltamalla olemassa olevaa tietoa. Monimuuttuja-analyysin pääpaino oli ferromagneettisten B-paikan suhteen järjestäytyneiden kaksoisperovskiittien etsimisessä. Ehdotetuilla yhdisteillä on käytännön kannalta sopiva ferromagneettinen siirtymälämpötila, ilman että yhdisteen sovellus vaatisi kohtuuttomia järjestelyitä. Työssä muutettiin kaksi ei-perovskiittirakenteista yhdistettä, (Sr,Ba)2FeSbO6 sekä Y2CuTiO6, kaksoisperovskiiteiksi, joiden kiderakennetta voitiin muuttaa niin normaali- kuin korkeapainesynteesimenetelmin. SIMCA-monimuuttuja-analyysiä käytettiin ferromagneettisten B-paikan suhteen järjestäytyneiden kaksoisperovskiittien analysoinnissa. Käytetyn mallin avulla löydettiin kokeellisen ja ennustetun siirtymälämpötilan välinen yhteys. Analyysin tuloksena löydettiin olemassa olevia ei-perovskiittirakenteisia yhdisteitä, aiemmin magneettisilta ominaisuuksiltaan määrittelemättömiä sekä uusia aiemmin tuntemattomia ferromagneettisia perovskiitteja

Assessment of magnetic properties of A2B′B′′O6 double perovskites by multivariate data analysis techniques

Multivariate data analysis is a promising tool for structure-property data mining and new-material prediction in the field of inorganic materials chemistry. Here we demonstrate its usability in assessing the magnetic properties of one of the most intriguing and plural functional inorganic material families, the ordered perovskite oxides of the A2B′B′′O6 type.Peer reviewe

High-pressure stabilisation of R = Y member of R2CuTiO6 double perovskite series

Publisher Copyright: © 2022 The Author(s)Double perovskite oxides of the A2B'B″O6 type with Jahn-Teller active Cu2+ as the B′ constituent have gained considerable research interest in recent years. For fundamental studies, the rare earth element (R) based systems such as R2CuTiO6 form an intriguing research platform as they allow systematic chemical-pressure studies by simply controlling the size of the R constituent. However, for the R2CuTiO6 compounds conventional ambient-pressure high-temperature synthesis yields an orthorhombic (Pnma) double perovskite structure for the largest R constituents (La–Gd) only, while the compounds with the smaller R:s adopt a hexagonal structure. Here we demonstrate a hexagonal-to-perovskite structure conversion for the R ​= ​Y compound achieved through a high-pressure (HP) high-temperature treatment at 4 ​GPa and 1000 ​°C. Structural details of the thus stabilized new double perovskite phase of Y2CuTiO6 are addressed through a combined DFT simulation and Rietveld refinement study, revealing signs towards the rare layered-type ordering of the B-site (Cu and Ti) cations. Similar to the previously reported R2CuTiO6 perovskite phases with R ​= ​La, Pr, and Nd, the R ​= ​Y member is found paramagnetic throughout the measured temperature range of 5–300 ​K. From UV–vis absorption measurements the optical bandgap is estimated to be ca 3.4 ​eV.Peer reviewe

Dynamical magnetic behavior of anisotropic spinel-structured ferrite for GHz technologies

We have fabricated a high quality magnetic Ni0.5Zn0.5Fe2O4 ferrite powder/polymer composite sheet consisting of common and environmentally friendly elements only. The sheet was then tested for its dynamic permeability by irradiating with electromagnetic waves with frequencies up to 50 GHz. Two different originally developed methods were used for the high-frequency permeability measurements, a short-circuited microstrip line method and a microstrip line-probe method. It is challenging to measure the dynamic permeability of magnetic thin films/sheets beyond 10 GHz because of the low response signal from these materials. However, the two methods produced essentially equivalent results. In the frequency dependent permeability profile, the maximum position of the profile, μmax″, shifted towards higher frequencies upon increasing an applied (strong) static external magnetic field, Hdc. A linear relationship between μmax″ and Hdc for the entire range of Hdc was observed even at small Hdc. In general, the spinel-structured Ni-based ferrites exhibit low magnetic anisotropy, but the present sample showed a uniaxial-anisotropic behavior in the parallel direction of the sheet. Our Ni0.5Zn0.5Fe2O4 powder/polymer composite sheet thus exhibits high performance at GHz frequencies, and should be applicable e.g. as an anisotropic electromagnetic wave-interference material.Peer reviewe