Crystal field studies of Ni2+ in α-Zn3(VO4)2

  • Published on
    26-Aug-2016

  • View
    213

  • Download
    0

Embed Size (px)

Transcript

  • Mat. Res. Bull. Vol. 3, pp. 585-594, 1968. Pergamon Press, Inc. Printed in the United States.

    CRYSTAL FIELD STUDIES OF Ni 2+ IN e-Zn3(V0a) ~

    D. K. Nath and F. A. Hummel ,~- Ceramic Science Section

    Materials Science Department The Pennsylvania State University

    University Park, Pennsylvania 16802

    (Received May 13~ 1968: Refereed)

    ABSTRACT Complete solid solubil ity was found in the system

    Ni3(VOs)2-a-Zn3(V0a) 2. Optical spectra of Ni 2+ dooed

    ~-Zn3(V04) samples were discussed in the light of

    crystal field theory. The calculated and theoretical frequencies were in good agreement for' octahedra! symmetry in the zinc sites. The crystal field parameter Dq was consistant with the ionic approximation rule. The Nephelauxetic ratio showed good resemblance with the cation-anion distance.

    Introduction

    Brown and Hummel (i) reported three polymorphic modif icstion

    of zinc orthovanadate.

    95o _zns(v0a) 2 2 : -Zn3 (V0a) 2

    Dur i f , Ber taut and Pauthenet (2) found that o r thovanadates o f the

    type M 32+(V05)2 where M 2+ = Hn 2+, Co 2+, Ni 2+ and Mg 2+ were

    i somorphous w i th ~-Zn3(V0&) 2. The s t ruc ture determinat ion 18 revea led or thorhombic symmetry w i th the space group D2h- ibam.

    *The writers are Research Associate and Professor and Chairman of the Ceramic Science Section of the Materials Science Department, The Pennsylvania State University, University Park, Pa. 16802. Contribution Number 67-23 from the College of Earth and Hinora! Sciences, The Pennsylvania State University, University Park, Pennsylvania, 16802.

    585

  • 586 CRYSTAL F IELD STUDIES Vol. 3, No. 7

    X-ray investigations revealed that the zinc and vanadium atoms

    occupy octahedral and tetrahedral positions respectively.

    In the present investigation, the mutual solid solubil ity

    of Ni 2+ in a-Zn3(V04) 2 and the respective site symmetry at the

    substitution centers were determined by x-ray powder diffraction

    and absorption spectra.

    Experimental

    The raw materials consisted of chemically pure NiC03, ZnO

    and V20 ~. The samples were wet mixed in acetone, heated to 550C

    for 24 hours, remixed in acetone and finally heated to 750C for

    48 hours.

    The phase identif ication was done by the petrographic

    microscope and a Norelco x-ray diffraetometer. The x-ray powder

    diffraction data were obtained by using Ni-f i ltered CuK~-radiation.

    A scanning rate of 2 (2~)/min. was maintained for routine x-ray

    diffraction analysis. For precise d-value measurements a silicon

    external standard and a scanning rate of 1/8 (2~)/min. were

    employed.

    The absorption spectra were recorded by a Beckman DK-2 type

    apparatus equipped with reflectance attachment. Since the samples

    were in the form of powders, a diffuse reflectance technique was

    employed in order to identify the structural characteristics of

    the transmission method, but no quantitative resolution of

    osci l lator strength or extinction coefficient could be made.

    Results and Discussion

    X-ray Investigation

    The results show complete solid solubil ity between Ni3(V04) 2

    and ~-Zn3(V04) 2. The variations of d-spacings as a function of

    composit ion are given in Figure i. In the solid solution series,

    ~-Zn3(VO%) 2 showed a continuous decrease of d-values with

    increasing proportion of Ni 2+. Comparing the ionic sizes of Ni 2+ o o

    (0.69 A) with that of Zn 2+ (0.7% A), a decrease in cell parameter

    of ~-Zn3(V0%) 2 seems to be logical.

    The plot of d-values against compositions for nickel incor-

    porated samples showed a slight positive deviation from Vegard's

    law. Spectrophotometric Investigation

    The absorption spectra for NixZn3_x(VOs) 2 samples (where

  • Vol. 3, No,. 7 CRYSTAL F IELD STUDIES 587

    x= 0.15, 2.0, 2.5) are given in Figure 2. The sharp absorpt ion

    peaks namely band (Vl) 7576 ~ 7813 cm -I and band (v 2) 12987 ~

    13514 cm -1 were observed in all the samples. Another absorpt ion

    region band (v3) could be observed near 22000 ~ 25000 cm -I The

    resolut ion of this peak posi t ion was not clear due to strong back-

    ground absorpt ion ar is ing out of tlhe charge transfer spectra of

    V 5+. Exper iments using samples di luted with Mg0 did not resolve

    the peak position. An extrapolat ion technique was employed to

    ascertain this band posi t ion precisely. 2~

    2..~

    2.,f:4 o~

    c 2.5,3

    252

    251

    I I I I, I I I I , I 2-E)O~ 90 80 70 60 ~ 40 30 20 I0 0

    ~a-Zn 3 (V04) 2 in (mole)

    FIG. i Plot of d-values as a funct ion of composit ion in the

    System Ni3(V04) 2 - c~-Zn3(V04) 2

    I,O

    0.8

    '" 0 .6 8

    "..7- 04 O. 0

    0.2

    - 3--rag ( F ) $Ti g.(F) 3Tig (P) I k I

    _ j ',,\ // v ,f

    /// ~ _ / ' - - / -x = 2 .5

    , I , I J I ~ I l I 2600 2200 1800 1400 I000 600 200

    Wave length m~

    FIG. 2 Absorpt ion spectra of NixZn3_xCVO 4_

    2

  • 588 CRYSTAL F IELD STUDIES Vol, 3, No. 7

    The work of Pappalardo, Wood and Linares (3) on the single

    crystals of NixMgl_xAl204 and NixZnl_x0 at 78 and 4.3K and of

    Schmitz-Dumont and Kasper (4) on nickel doped ilmenite show that

    the band near 4500 cm -I of [Ni2+] 4 does not interfere with the

    bands of [Ni2+] 6 and the band near 24600 cm -I of [Ni2+] 6 does not

    coincide with any band of [Ni2+] 4. Another characteristic diff-

    erence between spectra of [Ni2+] 4 and [Ni2+] 6 is exhibited by the

    relatively high intensities of the absorption bands of the former

    (5) (c.a. molar absorbance scale i-i00 for [Ni2+] 6 and ~ 200 for

    [Ni2+] 4 in the visible range). Furthermore the positions of the

    absorption maxima of octahedral Ni 2+ are shifted towards the UV

    region of the spectrum.

    A~ analysis of the calculated and observed frequencies and

    the band assignments are given in Table I showed exclusively the

    presence of octahedral Ni 2+. The complete energy level diagram

    as developed by Berkes and White (6) from exact solutions of the

    Tanabe-Sugano equations is reproduced in Figure 3. Actually

    because of weak spin-orbit coupling, the spin-forbidden transitions

    could not be observed in the present case, resulting in the trans-

    ition of the 3A2g (F) ground state to the three triplet excited

    states 3T2g (F), 3Tlg(F) and 3Tlg (P) only.

    50,O(

    40,0c

    30,0C A

    'E u

    c_

    h, 20,0(

    F IQ. 3

    Energy level diagram of Ni 2+ as a function

    of octahedral field

    strength Dq. Dq. of

    Ni0.15Zn2.85(V04)2 = -i 770 cm

    I O,OC

    0 - IOOO - 2OOO Dq in cm- J

    _ 300~A2g ( F )

  • B~

    nd

    v 1

    v V

    2

    v 3

    *Z3V

    2 :

    Ban

    d (c

    m -I

    )

    v 1

    V

    Z

    v 3

    v Dq

    B

    TAB

    LE i

    . B

    and

    posi

    tion

    s an

    d as

    sign

    men

    ts

    of

    Nix

    Zn3

    _x(V

    04)

    2 co

    mpo

    siti

    ons

    Nix

    Zn~

    b_x(

    V04)

    2,

    Obs

    erve

    d fr

    eque

    ncy

    (cm

    -I)

    Gro

    und

    Stat

    e 3A

    2g (

    F)

    Cal

    cula

    ted

    Tran

    siti

    on

    freq

    uenc

    y to

    (c

    m -I

    )

    3T

    (F)

    7700

    3

    2g

    3Tlg

    (F)

    12

    950

    Tlg

    (P)

    2395

    0

    x =

    0.

    15

    0.75

    1.

    0 2.

    0 2.

    5 2.

    8

    7692

    75

    76

    7692

    76

    92

    7752

    78

    13

    1315

    8 12

    987

    1298

    7 13

    333

    1333

    3 13

    514

    2400

    0 23

    300

    2280

    0 22

    470

    2222

    2 22

    222

    Col

    or

    Yello

    w

    Yello

    w

    Yello

    w

    Yello

    w

    Yello

    w

    Yello

    w

    Phas

    es*

    Z3V

    2 Z

    3V 2

    Z3V

    2 Z

    3V 2

    Z3V

    2 Z

    3V 2

    -Zn3

    (V04

    ) 2

    solid

    so

    luti

    on

    TAB

    LE 2

    . B

    and

    Ass

    ignm

    ents

    , ob

    serv

    ed

    freq

    uenc

    ies

    and

    crys

    tal

    field

    pa

    ram

    eter

    s of

    N~

    2+ i

    n M

    g0,

    a-Z

    n3(V

    04)

    2 an

    d C

    dTiO

    3

    Tran

    siti

    on

    to

    NIo

    .IM

    g0.9

    0 N

    io.1

    5Zn2

    .85(

    V04

    )2

    Ni0

    .01

    Cd

    0.~

    TiO

    3

    3 T

    3 2g

    (F

    ) 86

    00

    7692

    60

    00

    3Tlg

    (F

    ) i~

    800

    1315

    8 i0

    300

    1Tlg

    (P

    ) 24

    65O

    24

    OO

    O

    2O3O

    O

    Eg

    (D)

    1370

    0 14

    000*

    13

    000

    86

    0

    770

    600

    870

    92

    0

    830

    <

    O Z

    O f~

    U

    *cal

    cula

    ted

  • 590 CRYSTAL F IELD STUDIES Vol . 3, No . 7

    Assuming a weak field model, the energy separat ion of the

    di f ferent levels wil l be dependent on the crystal f ield strength

    Dq and Racah parameter B. These parameters were calculated from

    the Tanabe-Sugano matr ices (7) neglect ing the effect of spin-orbit

    coupling. The fol lowing equations were derived from the Tanabe-

    Sugano matr ices for d 8 electron in an octahedral field.

    v I = i0 Dq

    ~2 = 15 Dq + 7.5~ - 1 /2[ (10Dq)

    ~3 = 15 Dq + 7.5B + 1 /2[ (10Dq)

    B = 1/3 (v3 Vl) 5v 3 - 9v I

    2 + 225 B 2 - 180 Dq B] I/2

    2 + 225 B 2 - 180 Dq B] I/2

    B for the free ion state was calculated from the exper imental

    values given by Moore (8). The theoret ical energies of terms for

    d ~ conf igurat ion are given as E(3F) = A - 8B and E(3p) = A + 7B.

    A survey of the peak posit ions (cm-l), Dq and B values for Ni 0.01- Cd0.99Ti03 and Ni0.1Mg0.90 as given in the l i terature (5) are

    included in Table II a long with that of N i0 .15Zn2.85(V04)2 in

    order to compare the structural features of these three compounds.

    The spectra show that the band posit ions of NixZn3_x(V04) 2

    are shifted towards the IR port ion of the spectrum in comparison

    to Ni n 7Mgn a0. Since Ni 2+ is larger than Mg 2+ but smaller than

    Zn2+,Van-- exchange~'~ of Ni 2+ for Zn 2+ wil l produce an increase in the

    Ni-0 bond length. This expansion wil l be transmitted over the

    whole latt ice resul t ing in ~ decrease of the field parameter Dq at

    the subst i tut ion centers. Because the bands Vl, v2, and v 3 are

    either ent irely or part ia l ly dependant on Dq, the bands wil l be

    shifted towards longer wave lengths in NixZn3_x(V04) 2 corresponding

    to the widening of octahedral holes in comparison to the case of

    Mg0.

    A plot of the crystal f ield parameter Dq as a funct ion of the

    radius of the host ions Mg 2+ Zn 2+ and Cd 2+ is given in Figure 4

    The l inearity of the var iat ion of Dq with the host ion sizes is in

    good agreement with the inverse f i fth power re lat ionship of the

    ionic approx imat ion of the crystal field theory. Thus depending

    on the shrinkage or expansion, increase or decrease of Dq values

    were observed along the series Mg 2+ Z n 2 ~ 2+ + + Cd

    The values for the interelectronic interact ion term B which

    measures the degree of covalency between the transi t ion metal ion

  • Vol. 3, No. 7 CRYSTAL F IELD STUDIES 591

    FIG. 4

    Plot of Dq as a funct ion of the

    radius of the host ion.

    and its ligand, are plotted as a funct ion of the radius of the

    host ions, Hg 2+, Zn 2+ and Cd 2+ (F igure 5) . The Nephe lauxet ic

    ratio of Ni 2+ in Mg0 (0.77), a-Zn3(V04) 2 (0.81), and CdTi03 (C.7~)

    shows that covalent character of Ni-0 bond is increased by incor-

    porat ing Ni 2+ in Hg 2+ ins tead o f Zn 2+ s i tes . S tephens and

    Drickamer (9) observed a decrease of B when the mixed crystal

    NixKgl_x0 was placed under high pressure d iminish ing the Ni-0

    distance. The results on CdTi03 do not hold good in this respect.

    i -~ 900 ~D a ~k .

    "o 700

    LL

    Cdo.99TiO 3 >'600 o i

    [ I ooo p! I ' 08 0.9 10

    M 2+ Z 2+ Cd2+ Ahrens Ionic Radius in AngstrOms

    FIG. 5

    Plot of B as a funct ion of the

    radius of the host ion.

    IJSO

    I iO0

    E S

    } ~ooo E

    o n

    900 m

    r--

    o o ~: 8oo

    700

    -- B free ion Ni 2+ 1130cm - I

    Q

    _ N i0.15 Zn2.85 (VO 4 )2

    Nio j Mgo.gQ

    _ Nio.ol C, c10.99 TiO 3

    I ! J [ I I I 0.6 0 .7 0 .8 0 .9 1.0

    M 2+ Z 2~- Cd2+

    Ahrens Ionic Rad ius in ~ngst rSms

    The spectra of NixZn3_x(V0a) o showed the absence of the

    t r ip let -s ing let transit ion, 3A2g (F) iEg (D) which appeared

    as a shoulder in Ni0.1Mg0.90 (13700 cm -I) and as an obvious

  • 59Z CRYSTAL F IELD STUDIES Vol. 3, No. 7

    separate peak in Ni0.01Cd0.99Ti03 (13000 cm-l). According to

    quantum mechanical selection rules, such a transition correspond-

    ing to a state of different multipl icity will ordinarily be not

    observed due to weak spin-orbit interaction. A close inspection

    of the energy level diagram (Figure 3) shows that due to extreme

    closeness in energy at the Dq value given by 770 cm -I, 3Tlg (F)

    and IEg (D) states mix together in NixZn3_x (V04)2 compositions and

    appear only as a weak shoulder in Ni 0 iMg090 with a Dq value of - - i " "

    860 cm These two bands are well resolved in Ni0.01Cd0.99Ti03

    (Table II) because of low Dq value. The band v 2 of NixZn3_x(V04) 2

    composition became the most intense due to this superimposition

    effect. Other experimental evidences eg. Ni0.1Mg0.90 and

    Ni~ ~Cd~ ^~Ti0~ showed that the band v 3 corresponding to the u.u i u .~ 3 j 3

    transition A 2 (F) Tlg (P) was of the highest intensity. The g i energy of the level E (D), 13000 cm -I, in case of Ni Cd

    g 3Tlg _i0.01 0.99- Ti03 is higher than that of (F) level, 10300 cm , but this

    situation reverses in Ni0.1Mg0.90 due to the crossing of these two

    terms.

    The change in the maxima position of the third band v 3 with

    increasing proportion of Ni 2+ in NixZn3_x(V04) 2 did not reveal

    any systematic compositional effect on the shrinking of the crystal

    lattice. Comparatively, the bands v I and v 2 showed a gradual

    change toward longer wave lengths as would be expected with an

    expanding crystal volume resulting from a decrease of Ni 2+ concen-

    tration in a-Zn3(V04) 2. Acknowledgment

    The authors are grateful to the Ferro Corporation, Cleveland,

    Ohio, for the financial support which made this work possible.

    References

    i. J. J. Brown and F. A. Hummel, Trans. Brit. Cer. Soc., 6_~4, [9], 419, (1965).

    2. A. Durif, F. Bertaut and R. Pauthenet, Acta. Cryst., i_33, 1015, (1960) (in English).

    3. R. Pappalardo, D. L. Wood and R. C. Linares, Jr., J. Chem. Phys., 35___,[4], 1460, (1961).

    4. O. Schmitz-Dumont and H. Kasper, Monat. fur Chem., 95, [6], 1433, (1964) (in German).

    5. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, p. 736, Interscience Publishers, New York, (1962).

  • Vol. 3, No. 7 CRYSTAL F IELD STUDIES 593

    6. J. S. Berkes and W. B. White, Phys. and Chem. of Glasses, 7, [6],]91, (1966).

    7. Y. Tanabe and S. Sugano, J. Phys. Soc., Japan, ~ [5], 753, (1954) (in English).

    8. C. E. Moore, Atomic Energy Levels, U. S. Nat. Bur. of Stds. Circular No. 467, Vol. 2, U. S. Govt. Printing Office, Washington, D. C., (1952).

    9. D. R. Stephens and H. G. Drickamer, J. Chem. Phys., 34, [3], 937, (1961).