We report on H-1 NMR of (NixMn1-x)(3)[Cr(Cn)(6)](2) center dot nH(2)O

hexacyanochromates, where x changes from 0 to 1. The decay time

constants of the free induction decay signals described by an effective

spin-spin relaxation time T-2eff obtained from M(t) = M-0 exp(t/T-2eff)

decrease as the local magnetic moments increase produced by the

magnetic transition metal ions at the sites of the resonant H-1 nuclei.

The recovery of the magnetization in the spin-lattice relaxation time

(T-1) experiments was single-exponential

Bokor, Mónika Zsuzsanna

Kiss, László

Tompa, Kálmán

Zentko, A.

Zentkova, M.

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Vol. 113 (2008) ACTA PHYSICA POLONICA A No. 1Proceedings of the CSMAG’07 Conference, Kosˇice, July 9–12, 20071H NMR on(NixMn1−x)3[Cr(CN)6]2 · nH2OM. Bokor, K. Tompa, L.F. KissInstitute for Solid State Physics and Optics, HASP.O. Box 49, 1525 Budapest, HungaryM. Zentkova´, A. Zentko, M. Mihalik, S. Matˇasˇand Z. Mitro´ova´Institute of Experimental Physics, Slovak Academy of SciencesWatsonova 47, 040 01 Kosˇice, Slovak RepublicWe report on 1H NMR of (NixMn1−x)3[Cr(CN)6]2 · 15H2O hexa-cyanochromates, where x changes from 0 to 1. The decay time constantsof the free induction decay signals described by an effective spin–spin relax-ation time T2eff obtained from M(t) =M0 exp(t/T2eff) decrease as the localmagnetic moments increase produced by the magnetic transition metal ionsat the sites of the resonant 1H nuclei. The recovery of the magnetization inthe spin–lattice relaxation time (T1) experiments was single-exponential.PACS numbers: 72.80.Ga, 75.50.Xx, 76.60.Lz1. IntroductionMagnetic properties of prussian blue analogues (PBA) have been intensivelystudied in the last two decades. A large variety of attractive phenomena has beenobserved during this period, such as spin glass like behavior, photomagnetic be-havior, piezomagnetic behavior and magnetic pole inversion [1–5]. The simpleface centered cubic lattice of PBA’s is however far from being free of defects andvacancies, part of metal ion positions are vacant, while some of them are filledwith the water molecules. The water content, which can be modified during syn-thesis, influences significantly the resultant magnetic properties of prepared PBA,namely the value of the Curie temperature and the value of saturated magne-tization [6]. High level of disorder present in the PBA crystal lattice leads tothe increased importance of experimental techniques enabling to study the localmagnetic structure in investigated compounds. Magnetic local structures of dif-ferent PBA have been studied previously by solid state 13C and 2H NMR in [7, 8].(485)486 M. Bokor et al.It has been shown that the NMR spectrum of (Ni0.4Mn0.6)3[Cr(CN)6]2 · nD2O,is not a simple superposition of the two spectra of Mn3[Cr(CN)6]2 · nD2O andNi3[Cr(CN)6]2 ·nD2O . The crystal water molecules of the mixed system sense anaveraged hyperfine field of Ni and Mn ions and they are tightly bound neither to Ninor Mn ions at high temperatures [8]. In our paper we present the 1H NMR spec-tra for (NixMn1−x)3[Cr(CN)6]2 · nH2O at room temperature for different valuesof x.2. ExperimentalPolycrystalline (NixMn1−x)3[Cr(CN)6]2 · 15H2O hexacyanochromates (x =0, 0.32, 0.34, 0.38, 0.42, 0.46, 1) were prepared by mixing of K3[Cr(CN)6] andMnCl2 aqueous solutions. The fraction of x was varied from 0 to 1 by using ofequivalent volume of reactants. The Cr, Mn, and Ni amount was verified by atomicabsorption spectroscopy. The crystal structure symmetry was confirmed as cubic,space group Fm−3m (No. 225) by X-ray diffraction measurements. Magnetizationmeasurements were carried out with a SQUID magnetometer (MPMS). The protonmagnetic resonance measurements were accomplished by a Bruker SXP 4-100 pulseNMR spectrometer of ≈ 10−6 resolution at frequency of ν0 ≡ ω0/2pi = 82.55 MHz.1H free induction decay (FID) signals and spin echoes generated by the pulsesequence (pi/2)X−τ−(pi)Y were detected. Spin–lattice relaxation times (T1) weremeasured with the inversion-recovery method. Hydrogen content was determinedby extrapolation of the FID signal amplitudes to t = 0.3. ResultsSubstitution of Mn ions in the lattice by Ni leads to the decrease in theelementary cell volume (Fig. 1a). The presence of two magnetic sub-lattices, fer-romagnetic interaction between Ni and Cr and anti-ferromagnetic between Mn andCr, results in the occurrence of the so-called compensation temperature Tcomp, thatmeans the temperature at which the two magnetic sub-lattices are fully compen-sated. The position of the compensation temperature shifts with increasing x tolower values, and for x = 0.42 no Tcomp was observed (Fig. 1b). Figure 2a showsthat the compensation temperature decreases with increasing magnetic field. The1H nuclei in the water molecules served as local probes in the NMR experiments.The amplitude of the time-domain FID signal at t = 0 is proportional to the num-ber of 1H nuclei in the sample. This makes possible to determine the amount ofresonant water molecules i.e. the molar ratio nNMR/nformula (Table). The reso-nance frequency of the protons in water molecules near to the magnetic metal ionsis shifted by the local magnetic moments so far from the applied frequency thatthe signal of these protons is not detected and we obtain nNMR/nformula < 100%.The decay time constants of the FID signals described by an effective spin–spinrelaxation time T2eff obtained from M(t) = M0 exp(t/T2eff) decrease as the localmagnetic moments produced by the magnetic transition metal ions at the sites1H NMR on (NixMn1−x)3[Cr(CN)6]2 · nH2O 487Fig. 1. (a) The effect of x on the volume of the elementary cell. (b) The effect of x onthe compensation temperature Tcomp.Fig. 2. (a) The effect of magnetic field on compensation temperature. (b) Room--temperature 1H NMR spectra for (NixMn1−x)3[Cr(CN)6]2 · 15H2O compounds.of the resonant 1H nuclei. The recovery of the magnetization in the spin–latticerelaxation experiments was single-exponential (Table).Frequency-domain spectra were obtained by the Fourier transformation ofthe spin echo signals (Fig. 2b). The full widths at half maxima of them (Table)widens as the local magnetic moments produced by the magnetic transition metalions at the sites of the resonant 1H nuclei. The presence of more than one type ofresonant 1H nuclei belonging to different types of water molecules in the samplecan result in spectral widening also. The detectable 1H nuclei for the compoundNi3[Cr(CN)6]2 · 15H2O belong to metal ions at the sites of the resonant 1H nuclei.The detectable 1H nuclei for the compound Ni3[Cr(CN)6]2 ·15H2O belong to eitherthe coordinated or the zeolitic water molecules. We suppose that they belong mostprobably to the nickel-coordinated waters by the following reasons. (i) There aresix such water molecules per formula unit and their relative number of 6/15 = 0.4agrees with the measured ratio nNMR/nformula. (ii) Much higher ratio of 1H nucleiare detectable in the x < 1 compounds and practically all of the protons areresonant for Mn3[Cr(CN)6]2 ·15H2O (Table). This can be understood if we assume488 M. Bokor et al.TABLERoom-temperature 1H NMR parameters nNMR/nformula — relative amount of reso-nant 1H nuclei; T1 — spin–lattice relaxation time; T2eff — effective time constant ofFID signal; FWHM — full width at half maximum of the spectrum.Compound nNMR/nformula [%] T1 [ms] T2eff [µs] FWHM [kHz]Mn3[Cr(CN)6]2 92±10 0.65±0.03 15.2 19.0±0.1(Ni0.32Mn0.68)3[Cr(CN)6]2 66±10 0.86±0.04 14.0 20.1±0.1(Ni0.34Mn0.66)3[Cr(CN)6]2 74±10 0.79±0.04 14.2 20.2±0.1(Ni0.38Mn0.62)3[Cr(CN)6]2 80±10 0.85±0.04 15.6 18.0±0.1(Ni0.42Mn0.58)3[Cr(CN)6]2 75±10 0.83±0.04 14.7 19.2±0.1(Ni0.46Mn0.54)3[Cr(CN)6]2 70±10 0.78±0.04 13.7 20.3±0.1Ni3[Cr(CN)6]2 40±10 6.2±0.3 33.0 8.8±0.1average (0.32 ≤ x ≤ 0.46) 73 0.82 14.4 19.6that the magnetic moment of manganese ions and their magnetic interaction withchromium ions are dominant over the magnetic fields of nickel or chromium ionsand over the Ni–Cr interaction. As an overall result, the protons in the manganesecompounds experience more homogeneous spatial distribution of local magneticfield strength and nature than in Ni3[Cr(CN)6]2·15H2O. (iii) The NMR parametersT1, T2eff and FWHM show that detectable 1H nuclei in the manganese containingcompounds are subjects of significantly greater local magnetic moments than inNi3[Cr(CN)6]2 · 15H2O.AcknowledgmentsThis work was supported by APVT-20-005204, VEGA 2/7184/27 and by theHungarian Scientific Research Fund (project no. K62280).References[1] M. Zentkova´, M. Mihalik, J. Kovacˇ, A. Zentko, Z. Mitro´ova, M. Luka´cˇova´,V. Kavcˇansky´, L.F. Kiss, Phys. Status Solidi B 243, 272 (2006).[2] O. Sato, T. Iyoda, A. Fujishima, K. Hashimoto, Science 272, 704 (1996).[3] M. Zentkova´, Z. Arnold, J. Kamara´d, V. Kavcˇansky´, M. Luka´cˇova´, S. Mat’asˇ,M. Mihalik, Z. Mitro´ova´, A. Zentko, J. Phys., Condens. Matter 19, 266217 (2007).[4] D. Papanikolaou, W. Kosaka, S. Margadonna, H. Kagi, S. Ohkoshi, K. Prassides,J. Phys. Chem. C 111, 8086 (2007).[5] S. Ohkoshi, T. Iyoda, A. Fujishima, K. Hashimoto, Phys. Rev. B 56, 11642(1997).[6] S. Ohkoshi, K. Arai, Y. Sato, K. Hashimoto, Nature Mater. 3, 857 (2004).[7] S. Takeda, Y. Umehara, H. Hara, G. Maruta, Mol. Cryst. Liq. Cryst. 379, 253(2002).[8] H. Ishiyama, G. Maruta, T. Kobayashi, S. Takeda, Polyhedron 22, 1981 (2003).

H-1 NMR on (NixMn1-x)(3)[Cr(Cn)(6)](2) center dot NH(2)O

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