# Peak positions This chapter gives where each reflection lands on the 2θ axis, from the lattice metric, the wavelength and each aberration that shifts a peak. ## Lattice metric and Bragg’s law The d-spacing of reflection $(h,k,l)$ follows from the reciprocal metric tensor $G^*$ [[Shmueli, 2001](references.html.md#id18)]: $$ \frac{1}{d^2} \;=\; \mathbf{h} \cdot G^* \cdot \mathbf{h}^\top, \qquad G^* = G^{-1}, $$ *Source:* [`rietx.crystallography.lattice.inv_d_squared`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/crystallography/lattice.py#L53) where $\mathbf{h} = (h, k, l)$ is the row vector of Miller indices. $G$ is the direct metric tensor, the matrix of dot products of the cell edge vectors. It turns the six cell parameters into every length and angle the lattice has. $$ G \;=\; \begin{pmatrix} \mathbf{a}\cdot\mathbf{a} & \mathbf{a}\cdot\mathbf{b} & \mathbf{a}\cdot\mathbf{c} \\ \mathbf{b}\cdot\mathbf{a} & \mathbf{b}\cdot\mathbf{b} & \mathbf{b}\cdot\mathbf{c} \\ \mathbf{c}\cdot\mathbf{a} & \mathbf{c}\cdot\mathbf{b} & \mathbf{c}\cdot\mathbf{c} \end{pmatrix} \;=\; \begin{pmatrix} a^2 & ab\cos\gamma & ac\cos\beta \\ ab\cos\gamma & b^2 & bc\cos\alpha \\ ac\cos\beta & bc\cos\alpha & c^2 \end{pmatrix} \quad [\text{Å}^2]. $$ *Source:* [`rietx.crystallography.lattice.direct_metric_tensor`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/crystallography/lattice.py#L33) It is symmetric by construction, and its inverse $G^*$ is in Å⁻². A cubic cell makes $G = a^2 I$, and an orthogonal one makes it diagonal. Every off-diagonal element carries a cell angle away from 90°. Written out, with $(A, B, C, D, E, F) = (G^*_{11}, G^*_{22}, G^*_{33}, 2G^*_{23}, 2G^*_{13}, 2G^*_{12})$, [(2.1)](#equation-pos-dspacing) is the familiar quadratic form $$ \frac{1}{d^2} \;=\; A h^2 + B k^2 + C l^2 + D\,kl + E\,hl + F\,hk , $$ *Source:* [`rietx.crystallography.lattice.inv_d_squared`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/crystallography/lattice.py#L53) For an orthogonal cell ($\alpha = \beta = \gamma = 90°$) this reduces to $1/d^2 = h^2/a^2 + k^2/b^2 + l^2/c^2$, because the cross terms vanish with the off-diagonal elements of $G^*$. Peak positions then follow Bragg’s law, $$ 2\theta \;=\; 2 \arcsin\!\left(\frac{\lambda}{2d}\right). $$ *Source:* [`rietx.crystallography.lattice.two_theta_deg`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/crystallography/lattice.py#L143) Every emission line diffracts at its own Bragg angle. Differentiating [(2.4)](#equation-pos-bragg) at fixed $d$ gives the doublet-splitting law $$ \Delta 2\theta \;=\; 2 \tan\theta \cdot \frac{\Delta\lambda}{\lambda}, $$ *Source:* [`rietx.schemas.instrument`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/schemas/instrument.py#L1) The splitting grows with $\tan\theta$, so a Kα₂ line is not a fixed $2\theta$ offset from Kα₁. ### Time of flight A time-of-flight neutron bank does not scan an angle at all. It sits at one fixed scattering angle $2\theta_{\text{bank}}$, sees the whole moderator spectrum, and separates reflections by the time each neutron takes to fly from moderator to detector. Bragg’s law still holds channel by channel (a neutron arriving at time $T$ had wavelength $\lambda = 2 d \sin\theta_{\text{bank}}$), but the *position* of a reflection is a flight time, and the map from $d$ to $T$ is a calibration of the instrument rather than a consequence of one number [[Larson and Von Dreele, 2004](references.html.md#id15)]: $$ T(d) \;=\; \mathrm{DIFC}\, d \;+\; \mathrm{DIFA}\, d^{2} \;+\; \mathrm{TZERO} \;+\; \frac{\mathrm{DIFB}}{d}. $$ *Source:* [`rietx.schemas.instrument.TOFSource.tof_from_d`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/schemas/instrument.py#L773) $\mathrm{DIFC}$ (µs Å⁻¹) is the geometric term, $252.777 \, L \, 2 \sin\theta_{\text{bank}}$ for a total flight path $L$ in metres; $\mathrm{DIFA}$ (µs Å⁻²) is a small empirical curvature absorbing sample-position and detector-depth effects; $\mathrm{TZERO}$ (µs) is the electronic time offset. The fourth term is GSAS-II’s `difB` (µs Å), absent from the GSAS manual’s relation and zero in most projects, carried because a reader built to the three-term form would drop a non-zero one *silently*. The wavelength–cell degeneracy below reappears here as $\mathrm{DIFC}$ against the cell: $T \propto \mathrm{DIFC}\, d$ fixes only the product, so a standard refinement holds the cell at its certified value and frees $\mathrm{DIFC}$, and a sample refinement does the reverse. Freeing both on one bank is the same flat direction with different letters. ### Monochromator harmonics A crystal monochromator set to pass $\lambda$ by Bragg reflection from planes of spacing $d_M$ satisfies $\lambda = 2 d_M \sin\theta_M$. At that same setting $$ \frac{\lambda}{n} \;=\; 2\,\frac{d_M}{n}\,\sin\theta_M , $$ *Source:* [`rietx.schemas.instrument.Harmonic`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/schemas/instrument.py#L120) and $d_M/n$ is the spacing of the $n$th-order reflection of the same planes. So the transmitted beam carries $\lambda/n$ for every integer $n \ge 2$ whose reflection $n\cdot(hkl)$ is allowed. The order belongs to the reflection and not to the geometry, so no adjustment of $\theta_M$ removes it. That component diffracts from the specimen too. At detector angle $2\theta$ it satisfies $\lambda/n = 2 d \sin\theta$, so it is diffracting from planes of spacing $$ d \;=\; \frac{\lambda}{2 n \sin\theta} \qquad\Longleftrightarrow\qquad \sin\theta_n \;=\; \frac{1}{n}\,\sin\theta_1 , $$ *Source:* [`rietx.schemas.instrument.Harmonic`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/schemas/instrument.py#L120) where $\theta_1$ and $\theta_n$ are where the fundamental and the harmonic put the same reflection. Since $\sin$ increases on $(0°, 90°)$, $\theta_n < \theta_1$, and the harmonic’s peak from a given $hkl$ sits at lower $2\theta$ than the fundamental’s. A factor on $\sin\theta$ relates the two, in the same structure as [(2.5)](#equation-pos-doublet) with a ratio of $n$ in place of a small $\Delta\lambda$. Two consequences decide the implementation. First, the harmonic diffracts the same $hkl$ list with the same $|F|^2$, because $|F|^2$ is evaluated at $\sin\theta/\lambda = 1/2d$, a property of the reflection alone. A $\lambda/n$ component is therefore an emission line here rather than a second phase. A phase with a doubled cell reproduces the positions ($2a$ has $d' = 2d$, and $\lambda$ at $d'$ lands where $\lambda/2$ at $d$ does), but it carries the structure factors of a fictitious cell, so its intensities are wrong. Second, $\lambda/n$ has $n$ times the Ewald radius, so the reflection list is generated for the shortest wavelength in the source. A list generated for the fundamental leaves the harmonic’s high-angle reflections silently absent. Whether a harmonic exists at all is arithmetic on the monochromator’s own structure. Copper is face-centred cubic with one atom per lattice point, so Cu(311) doubles to the fully allowed (622) and the second order passes. A diamond-structure crystal cut on all-odd indices cancels its own second order: the doubled indices are all even, and the diamond structure factor $1 + \exp[2\pi i (h+k+l)/4]$ vanishes unless $h+k+l \equiv 0 \pmod 4$, while doubling an all-odd triple always gives $h+k+l \equiv 2$. ### Satellites of a propagation vector A phase that orders with a commensurate propagation vector $\mathbf{k}$ scatters at $\mathbf{Q} = \mathbf{H} \pm \mathbf{k}$ for every vector $\mathbf{H}$ of its nuclear reciprocal lattice [[Rodríguez-Carvajal, 1993](references.html.md#id136)]. Positions follow from [(2.1)](#equation-pos-dspacing) with $\mathbf{h}$ replaced by $$ \mathbf{Q}_{\mathbf{H},m} \;=\; \mathbf{H} + m\,\mathbf{k}, \qquad m \in \{-1,\,0,\,+1\}, $$ *Source:* [`rietx.crystallography.satellites`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/crystallography/satellites.py#L1) so $m = 0$ is the nuclear reflection and $m = \pm 1$ its two satellites. The d-spacing, and hence [(2.4)](#equation-pos-bragg), is otherwise unchanged: a satellite is a position, and needs no moment, no magnetic form factor and no magnetic symmetry to be placed. Two rules decide which $\mathbf{Q}$ exist, and an implementation can get either wrong. First, $\mathbf{k}$ and $-\mathbf{k}$ are one vector exactly when $$ 2\mathbf{k} \in \Lambda^*, $$ *Source:* [`rietx.crystallography.satellites`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/crystallography/satellites.py#L1) with $\Lambda^*$ the reciprocal lattice, which is a *sublattice* of the integer $hkl$ grid whenever the direct lattice is centred. On a C lattice $(\tfrac12, 0, 0)$ and $(-\tfrac12, 0, 0)$ are therefore distinct, because $(1,0,0)$ violates $h + k = 2n$, while $(0, 0, \tfrac12)$ is a single vector because $(0,0,1)$ is a lattice point. Second, $\mathbf{H}$ runs over $\Lambda^*$ itself: the centring conditions decide which $\mathbf{H}$ exist, but a glide or a screw absence is a condition on the *nuclear structure factor*, and a satellite has no nuclear structure factor to be absent from: applying those conditions to a satellite drops reflections that are there. The multiplicity of a satellite is the number of distinct $\mathbf{Q}$ in its orbit under the Laue group, Friedel mates merged, with the reciprocal-space action $R^\top$. It is obtained by enumeration: the plausible closed form $|{\rm orbit}(\mathbf{H})| \times |{\rm star}(\mathbf{k})|$ is not even a bound, because $R^\top\mathbf{H}$ and $R^\top\mathbf{k}$ are images under the *same* $R$ and because reducing an arm of the star back into the first cell shifts the parent by a lattice vector. ## The wavelength–cell degeneracy [(2.4)](#equation-pos-bragg) reaches the wavelength only through the ratio $\lambda/2d$, so scaling $\lambda$ and every reciprocal-lattice length by the same factor leaves every computed position unchanged: $$ \lambda \to s\lambda, \quad \mathbf{a}^{*}_i \to s\,\mathbf{a}^{*}_i \;\;\Longrightarrow\;\; 2\theta_{hkl} \;\text{unchanged for all}\; hkl . $$ *Source:* [`rietx.params.vector.check_wavelength_freedom`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/params/vector.py#L101) For one histogram that one-parameter family is an exactly flat direction of the residual, whatever the data quality, so $\lambda$ and the cell cannot both be free. Differentiating [(2.4)](#equation-pos-bragg) in $\lambda$ gives $$ \frac{\partial\, 2\theta}{\partial \lambda} \;=\; \frac{2\tan\theta}{\lambda}, $$ *Source:* [`rietx.model.forward`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/model/forward.py#L1) the same $\tan\theta$ signature as [(2.5)](#equation-pos-doublet), and the signature a uniform cell scaling has. Across $N$ histograms of one specimen the cell is one object and the $\lambda_i$ are $N$ separate ones, so the family [(2.11)](#equation-pos-lambda-cell) collapses to a single scalar $s$. Fixing one $\lambda_i$ fixes $s$, and the other $N-1$ become measurable against the shared lattice. One wavelength is held, and at most $N-1$ are free. ## Aberration shifts rietx models additive $2\theta$ shifts that have distinct angular signatures. The signatures separate them, and only barely (see the decorrelation workflow below). Which shifts exist depends on the geometry, because each one is derived for a single specimen shape. The two under the zero-point error are flat-plate aberrations, and the capillary has its own pair further down. ### Zero-point error The zero-point error is a constant shift, and the only aberration common to every geometry. ### Sample displacement In Bragg-Brentano geometry a flat specimen whose surface sits a distance $s$ off the goniometer axis shifts every line. Only the ratio of $s$ to the goniometer radius $R$ enters, and both are in mm [[Klug and Alexander, 1974](references.html.md#id27); [Wilson, 1963](references.html.md#id26)]: $$ \Delta 2\theta \;=\; -\frac{2 s}{R} \cos\theta \quad [\mathrm{rad}]. $$ *Source:* [`rietx.model.corrections.displacement_shift_deg`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/model/corrections.py#L92) The $\cos\theta$ dependence separates it from the zero-point error. ### Sample transparency Finite beam penetration puts the effective diffracting surface below the physical one (thick-sample limit [[Klug and Alexander, 1974](references.html.md#id27); [Wilson, 1963](references.html.md#id26)]): $$ \Delta 2\theta \;=\; -t \sin 2\theta \quad [\mathrm{rad}], \qquad t = \frac{1}{2 \mu_{\mathrm{eff}} R}, $$ *Source:* [`rietx.model.corrections.transparency_shift_deg`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/model/corrections.py#L157) with $t \ge 0$ dimensionless. For strongly absorbing samples $t \to 0$ and the correction vanishes. ### Decorrelating the flat-plate trio These three columns (constant, $\cos\theta$, $\sin 2\theta$) are nearly collinear over a typical angular range, and all three trade against the cell parameters. Decorrelate them by calibration. Refine zero and displacement on a standard whose certified cell is held fixed, save the instrument profile, and load it frozen for sample work. ### Capillary displacement The Debye-Scherrer counterpart applies to a capillary whose diffracting volume sits off the centre of the $2\theta$ circle [[McCusker *et al.*, 1999](references.html.md#id9)]: $$ \Delta 2\theta \;=\; \frac{-a \sin 2\theta + b \cos 2\theta}{R} \quad [\mathrm{rad}]. $$ *Source:* [`rietx.model.corrections.capillary_displacement_shift_deg`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/model/corrections.py#L109) Here $a$ is the displacement along the incident beam, positive downstream, and $b$ the displacement perpendicular to it in the diffraction plane, positive toward increasing $2\theta$. Both are in mm, like $R$. The paper prints the same expression as $(x \sin 2\theta - y \cos 2\theta)/R$ and draws no axes. The signs above are fixed by derivation, and other codes attach the letter $x$ to the other term, so match the shapes and not the letters. Both terms are zero when the capillary is centred, and both are held fixed unless the geometry declares $R$. The trio for this geometry is (constant, $\sin 2\theta$, $\cos 2\theta$), and it is separable to the same limited degree. Over $5$–$160°$ the smallest eigenvalue of the unit-column Gram matrix is $5.2 \times 10^{-2}$, and over $5$–$25°$ it is $1.1 \times 10^{-5}$. ## Wavelength scales Kα₁/Kα₂ wavelengths here are peak positions of the measured line shapes, and not centroids. They are quoted on one consistent scale, the NIST X-ray Transition Energies Database [[Deslattes *et al.*, 2003](references.html.md#id22); [NIST, 2005](references.html.md#id24)]. Its 3d-metal values derive from the Hölzer et al. measurements [[Hölzer *et al.*, 1997](references.html.md#id23)] and its Mo/Ag values from Deslattes & Kessler [[Deslattes and Kessler, 1985](references.html.md#id25)], so the claim is one column of one evaluation and not one paper. Use that column for every anode. Mixing wavelength scales between anodes, or against an older table, is the classic ~100 ppm cell-parameter error. Bearden’s compilation [[Bearden, 1967](references.html.md#id21)] is a different scale, with Mo Kα₂ differing by 24 ppm. Correcting individual rows toward it reintroduces the same error. *Source:* [`rietx.schemas.instrument`](https://github.com/yue-here/rietx/blob/v1.10.0/src/rietx/schemas/instrument.py#L1)