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Predmet rada ove doktorske disertacije usmeren je na detaljno ispitivanje strukturnih, morfoloških, i magnetnih svojstava različitih polimorfa mangan-dioksida sintetisanih hidrotermalnom metodom. Proučavani su polimorfi β-MnO2 tetragonalne rutilne strukture i α-KxMnO2 tetragonalne holanditne strukture. Ispitivani materijali dobijeni su različitim postupcima sinteze i oni su ispitivani rendgenostrukturnom analizom, skenirajućom elektronskom mikroskopijom, metodom energetski disperzivne spektrometrije, ramanskom i infracrvenom spektroskopijom, metodom elektronske paramagnetne rezonancije i primenom merenja magnetizacije pomoću SQUID-magnetometra u konstantnom (DC) i promenljivom (AC) spoljašnjem magnetnom polju. Na osnovu merenja skenirajuće elektronske mikroskopije utvrĎeno je da svi ispitivani materijali poseduju morfologiju nanoštapića, što je potvrĎeno i mikrostrukturnom karakterizacijom na osnovu rendgenostrukturnih merenja. Na osnovu merenja vibracione spektroskopije potvrĎene su strukturne karakteristike ispitivanih materijala. Kod β-MnO2 materijala koji poseduje jednostavniju rutilnu strukturu, infracrvenom spektroskopijom detektovana je dodatna traka koja potiče od čestica drugačije veličine i/ili oblika, dok su ramanskom spektroskopijom uspešno detektovane dve dodatne trake koje potiču od strukturno neureĎenog i nestehiometrijskog regiona MnO2-δ na površini nanoštapića. PotvrĎeno je da β-MnO2 materijal ima antiferomagnetni prelaz na 93 K, a za α-KxMnO2 materijale ustanovljeno je kompleksnije niskotemperatursko ponašanje. α-KxMnO2 materijali holanditne strukture ispoljavaju reentrant ponašanje spinskog stakla: stanje spinskog stakla se javlja na temperaturama niţim od temperature na kojoj se javlja slabo feromagnetno ureĎenje. Stanje superspinskog stakla na niskim temperaturama kod oba α-KxMnO2 materijala nedvosmisleno je potvrĎeno kroz ispitivanje magnetnih relaksacija, starenja i memorijskih efekata. Merenja elektronske paramagnetne rezonancije kod β-MnO2 i α-K0,15MnO2 materijala pokazala su prisustvo dva tipa Mn4+ jona koja se nalaze u različitom lokalnom katjonskom okruţenju: (i) Mn4+ joni u čijem su lokalnom katjonskom okruţenju uglavnom samo Mn4+ joni i (ii) Mn4+ joni oko kojih je mešovito Mn4+/Mn3+ okruţenje. Region sa mešovitom valencom mangana kod β-MnO2 materijala potiče od nestehiometrijskog sloja MnO2-δ na površini nanoštapića, a kod α-K0,15MnO2 materijala, lokalno katjonsko okruţenje sa mešovitom valencom mangana javlja se u neposrednoj blizini tunelnih kalijumovih katjona. Rezultati dobijeni u ovoj disertaciji značajni su za bolje razumevanje strukturnih i magnetnih karakteristika nanostrukturnih mangan-dioksida, naročito nedovoljno istraţenih magnetnih karakteristika α-KxMnO2 holanditne strukture. The research topic of this doctoral dissertation is a detailed study of structural, morphological and magnetic properties of different polymorphs of manganese dioxides that are hydrothermally synthesized. Polymorphs β-MnO2 with tetragonal rutile-type structure and α-KxMnO2 with tetragonal hollandite-type structure were studied. Investigated materials have been obtained by different synthesis pathways and they were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman and infrared spectroscopy, electron paramagnetic resonance (EPR) and DC and AC magnetization measurements. SEM images showed nanorod shape of all specimens, while microstructure characterization done by XRD measurements revealed that crystallites have an elongated rod-like shape and hence, it confirmed the anisotropic morphology. Based on vibrational spectroscopic measurements, structural characteristics of investigated materials have been confirmed. In the infrared spectrum of β-MnO2 with a simpler rutile-type structure, the additional band that originates from particles of different shape and/or size has been obtained, while in the Raman spectrum, additional bands that originate from the presence of nonstoichiometric and disordered MnO2-δ on the surface of nanorods, have been obtained. Magnetic ordering of β-MnO2 is antiferromagnetic below 93 K, while α-KxMnO2 materials possess more complicated low-temperature behaviour of the reentrant spin-glass type: the onset of the spin-glass state occurs at temperatures below the weak ferromagnetic ordering. Low-temperature superspin-glass state has been confirmed through the investigations of magnetic relaxations, aging and memory effects. EPR measurements of β-MnO2 and α-K0.15MnO2 show the appearance of two types of Mn4+ ions having different local cationic environment: (i) Mn4+ ions in Mn4+-rich environment and (ii) Mn4+ ions in mixed-valence Mn4+/Mn3+ environment. For β-MnO2 material mixed-valence Mn4+/Mn3+ environment match up with nonstoichiometric MnO2-δ at the surface of the nanorods, while for α-K0.15MnO2 material local mixed-valence Mn4+/Mn3+ environment appears close to the tunnel potassium cations. The results obtained in this dissertation are important for a better understanding of the structural and magnetic properties of nanostructured manganese dioxides, in particular the insufficiently investigated magnetic properties of the α-KxMnO2 with holandite-type structure.

In this work, the polycrystalline samples Y1-xYbxF3 (x = 0.01, 0.05, 0.1, 0.25, 0.5, 0.75 and 1) were synthesized and characterized by X-ray powder diffraction and magnetic measurements. In the first stage of synthesis, mixed sesquioxides (Y1-xYbx)2O3 were obtained by ceramic technology. In the second stage, obtained mixed sesquioxides were treated with an excess of ammonium hydrogen fluoride in air at 170 °C for 20 h. In the third and last stage, Y1-xYbxF3 samples were obtained by heating the product from the second stage of synthesis at 500 °C for 3 h in an inert atmosphere. The X-ray diffraction revealed that the crystal structure of the Y1-xYbxF3 solid solution is orthorhombic (YF3 structural type). The structure refinements were done by the Rietveld full-profile method within Pnma space group. Magnetic susceptibility measurements of all samples were done in a temperature range 2-300 K by SQUID magnetometer. The relation between magnetic and structural properties was discussed.