The rare-earth centers in CsCdBr3

CsCdBr3 crystallizes in the quasi-linear CsNiCl3 structure. It possesses linear chains of [CdBr6]4- octahedra separated by parallel chains of Cs+. Trivalent rare-earth (RE) ions substitute for the divalent Cd ions. The need for charge compensation leads to a number of RE centers, the most prominent of which is the symmetric pair center RE3+-(Cd vacancy)-RE3+. Madelung calculations were performed for a number of different centers giving a ranking for their chance of realization. Experimental evidence from optical spectroscopy and submillimeter ESR is given for the most likely centers. The symmetric pair center is of special interest for cooperative phenomena of RE3+ ions. This includes direct ion-ion interactions and interactions with or via the electronic excitations of the host lattice. Some experiments are discussed. © 2002 Elsevier Science B.V. All rights reserved

Local lattice structure, crystal field and energy level patterns in CsCdBr3:Tm3+ crystals

In CsCdBr3, Tm3+ substitutes for Cd2+. It predominately forms symmetric dimer centers and single-ion centers, both of trigonal symmetry. The energy level schemes of both centers were determined by EPR and site-selective laser spectroscopy. To describe the spectra term dependent crystal-field parameters were deduced on the basis of a microscopic model taking into account the local lattice deformation induced by the impurity centers and the quasi-resonant virtual scattering of intrinsic lattice excitations by the Tm3+ ions. © 1998 Elsevier Science S.A

Initial growth of fourteen native tree species in response to potassium fertilization

Conduziu-se um experimento em casa de vegetação, objetivando avaliar a resposta ao potássio por quatorze espécies florestais nativas na fase de mudas. As espécies estudadas, agrupadas de acordo com o seu grupo sucessional, foram as seguintes: Pioneiras: Peltophorum dubium (Spreng) Taub., Senna spectabilis (DC) I.&B. (Spreng), Senna multijuga (L.C. Rich) I.&B., Stenolobium stans (Jun.) Seem, Jacaranda mimosaefolia D. Don., Guazuma ulmifolia Lam. e Trema micrantha Bloom; Secundárias: Anadenanthera peregrina (L.) Speg., Machaerium nictitans Vell. Benth., Cedrella fissilis Vell. e Senna macranthera (Collad.) I.&B.; Espécies Clímax: Copaifera langsdorffii Desf., Hymenaea courbaril L. var. stilbocarpa (Hayne) Lee et Lang, e Platycianus regnelli Benth. L.; Anual: Zea maiz L. Utilizou-se um solo com 0,72 mmolcK+.dm-3 pelo Mehlich-1, estabelecendo-se dois tratamentos: sem aplicação de potássio (-K) e com aplicação de 84 mg de K/kg de solo (+K). Em ambos os tratamentos, o solo recebeu os demais nutrientes e calcário. Observou-se uma resposta diferenciada à adubação potássica entre as espécies e grupos sucessionais. Com exceção da cássia-carnaval (Senna spectabilis), todas as demais espécies pioneiras tiveram incrementos significativos na produção de matéria seca total com a aplicação de potássio. No grupo das espécies secundárias, apenas o cedro (Cedrella fissilis Vell.) e o fedegoso (Senna macranthera) responderam à aplicação de potássio, enquanto as espécies clímax não foram responsivas. Tanto nas espécies pioneiras quanto nas espécies secundárias, as maiores respostas foram encontradas nas de maiores taxas de crescimento.An experiment was carried out under greenhouse conditions, in order to evaluate the potassium response of fourteen native tree species. The studied species, according to its ecological group were: Pioneer species: Peltophorum dubium (Spreng) Taub., Senna spectabilis (DC) I.&B. (Spreng), Senna multijuga (L.C. Rich) I.&B., Stenolobium stans (Jun.) Seem, Jacaranda mimosaefolia D. Don., Guazuma ulmifolia Lam. and Trema micrantha Bloom; Secundárias: Anadenanthera peregrina (L.) Speg., Machaerium nictitans Vell. Benth., Cedrella fissilis Vell. and Senna macranthera (Collad.) I.&B.; Espécies Clímax: Copaifera langsdorffii Desf., Hymenaea courbaril L. var. stilbocarpa (Hayne) Lee et Lang, and Platycianus regnelli Benth. L.; Anual: Zea maiz L. A soil with 0.72 mmolcK+.dm-3 soil by Mehlich-1 was used, establishing the following treatments: without potassium (-K), and with application of 84 mg K+/kg soil. In both treatments, soil was previously limed and conveniently supplied with other nutrients. Differential response was observed among species and ecological groups to the potassium fertilization. With exception of cassia-carnaval (Senna spectabilis), in all other pioneer species the total dry matter production was significantly increased with potassium fertilization. In the group of secondary species, only cedro (Cedrella fissilis Vell.) and fedegoso (Senna macranthera (Collad.) I.&B.) were responsive to potassium application, while climax species did not. The highest responses were found for pioneer and secondary species with fast growth rate

Submillimeter EPR spectroscopy of lanthanide compounds: Pair centers of Ho3+ in CsCdBr3

High-frequency EPR spectra corresponding to magnetic dipole transitions between the doublet-singlet-doublet group of the lowest crystal field levels of the trigonal single ion and the symmetric dimer centers in CsCdBr3 have been studied. For the dimer center, the absorption spectra in the manifold of the lowest 1024 electron-nuclear states are simulated by successive diagonalization of the electron-nuclear Hamiltonian which includes the crystal field, the electronic Zeeman energies, the magnetic and quadrupole hyperfine interactions, and the dipole-dipole interactions between the paramagnetic ions. The crystal field parameters of Ho3+ ions in the symmetric dimer are determined. © 1998 Elsevier Science S.A

Submillimeter electron-nuclear excitation spectra in CsCdBr3:Ln3+ (Ln=Tm, Ho) crystals

The ESR spectra of single and pair impurity centers of thulium and holmium ions in CsCdBr3:Tm3+ and CsCdBr3:Ho3+ crystals are measured in the frequency range 160-400 GHz. Analysis of the characteristic features of the hyperfine structure of the ESR lines and analysis of the variations in the spectra as a function of the temperature and external magnetic field shows that the Ln3+ ions substitute for Cd2+ ions and predominantly form symmetric pair centers of the type Ln3+-(vacancy at a neighboring Cd2+ site)-Ln3+. The ESR spectra of CsCdBr3:Ln3+ crystals are used to make a positive identification of the optical spectra of selective laser excitation. © 1997 American Institute of Physics