diff --git a/pixi.lock b/pixi.lock
index f45289f91..1d520d7da 100644
--- a/pixi.lock
+++ b/pixi.lock
@@ -9665,6 +9665,7 @@ packages:
   - diffpy-pdffit2
   - diffpy-utils
   - emcee
+  - filelock
   - gemmi
   - h5py
   - lmfit
diff --git a/pyproject.toml b/pyproject.toml
index 50e2a410e..57c7c712f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -26,6 +26,7 @@ dependencies = [
   'numpy',            # Numerical computing library
   'asciichartpy',     # ASCII charts for terminal output
   'pooch',            # Data downloader
+  'filelock',         # Cross-process locking for shared download caches
   'typer',            # Command-line interface creation
   'rich',             # Rich text and beautiful formatting in the terminal
   'varname',          # Variable name introspection
diff --git a/src/easydiffraction/analysis/calculators/crysfml.py b/src/easydiffraction/analysis/calculators/crysfml.py
index e3fae3c30..6520126b3 100644
--- a/src/easydiffraction/analysis/calculators/crysfml.py
+++ b/src/easydiffraction/analysis/calculators/crysfml.py
@@ -24,7 +24,7 @@
 
 from __future__ import annotations
 
-import string
+import re
 from typing import TYPE_CHECKING
 
 import numpy as np
@@ -60,23 +60,26 @@
 
 def _element_symbol(type_symbol: str) -> str:
     """
-    Strip a leading isotope number from an atom type symbol.
+    Extract the element from an isotope or ionic atom type symbol.
 
     CrysFML resolves scattering by element and does not understand
-    isotope prefixes such as ``11B`` or ``2H`` (cryspy does). Returning
-    the bare element symbol lets one model drive both engines.
+    isotope prefixes such as ``11B`` or ionic suffixes such as ``Fe3+``
+    (cryspy does). Returning the bare element symbol lets one model
+    drive both engines.
 
     Parameters
     ----------
     type_symbol : str
-        Atom type symbol, optionally isotope-prefixed (e.g. ``11B``).
+        Atom type symbol, optionally isotope-prefixed or charged (e.g.
+        ``11B`` or ``Fe3+``).
 
     Returns
     -------
     str
-        The symbol with any leading digits removed (e.g. ``B``).
+        The bare element symbol (e.g. ``B`` or ``Fe``).
     """
-    return type_symbol.lstrip(string.digits)
+    match = re.fullmatch(r'\d*([A-Z][a-z]?)(?:[1-8][+-])?', type_symbol.strip())
+    return match.group(1) if match else type_symbol
 
 
 def _cfl_label(name: str) -> str:
diff --git a/src/easydiffraction/datablocks/structure/categories/atom_sites/default.py b/src/easydiffraction/datablocks/structure/categories/atom_sites/default.py
index 3c6ed869e..0940ac708 100644
--- a/src/easydiffraction/datablocks/structure/categories/atom_sites/default.py
+++ b/src/easydiffraction/datablocks/structure/categories/atom_sites/default.py
@@ -160,7 +160,10 @@ def __init__(self) -> None:
             value_spec=AttributeSpec(default=None, allow_none=True),
             tags=TagSpec(
                 edi_names=['_atom_site.multiplicity'],
-                cif_names=['_atom_site.site_symmetry_multiplicity'],
+                cif_names=[
+                    '_atom_site.site_symmetry_multiplicity',
+                    '_atom_site_symmetry_multiplicity',
+                ],
             ),
         )
         self._occupancy = Parameter(
@@ -215,14 +218,22 @@ def __init__(self) -> None:
     @property
     def _type_symbol_allowed_values(self) -> list[str]:
         """
-        Chemical symbols accepted by *cryspy*.
+        Chemical and ionic symbols accepted by *cryspy*.
 
         Returns
         -------
         list[str]
-            Unique element/isotope symbols from the database.
-        """
-        return list({key[1] for key in DATABASE['Isotopes']})
+            Unique element/isotope symbols from the database, with
+            common signed oxidation-state suffixes.
+        """
+        symbols = {key[1] for key in DATABASE['Isotopes']}
+        ions = {
+            f'{symbol}{charge}{sign}'
+            for symbol in symbols
+            for charge in range(1, 9)
+            for sign in ('+', '-')
+        }
+        return list(symbols | ions)
 
     def _resolve_structure_space_group(self) -> object | None:
         """
diff --git a/src/easydiffraction/io/cif/handler.py b/src/easydiffraction/io/cif/handler.py
index 3166720f2..0a2a7316d 100644
--- a/src/easydiffraction/io/cif/handler.py
+++ b/src/easydiffraction/io/cif/handler.py
@@ -79,13 +79,23 @@ def cif_name(self) -> str:
 
     @property
     def cif_read_names(self) -> list[str]:
-        """Accepted ``.cif`` import names, in lookup order."""
-        return list(dict.fromkeys(self.cif_names))
+        """
+        Accepted ``.cif`` import names, in lookup order.
+
+        CIF dictionaries use both ``_category.item`` and the older
+        ``_category_item`` spelling.  Gemmi preserves the spelling from
+        the input document, so add the underscore form of every dotted
+        name as an import-only alias.  Explicitly declared names retain
+        priority over inferred aliases.
+        """
+        names = list(dict.fromkeys(self.cif_names))
+        aliases = [name.replace('.', '_', 1) for name in names if '.' in name]
+        return list(dict.fromkeys([*names, *aliases]))
 
     @property
     def read_names(self) -> list[str]:
         """Names accepted on read across both formats (union)."""
-        return list(dict.fromkeys([self.edi_name, *self._edi_names, *self.cif_names]))
+        return list(dict.fromkeys([*self.edi_read_names, *self.cif_read_names]))
 
     @property
     def category_name(self) -> str:
diff --git a/src/easydiffraction/utils/utils.py b/src/easydiffraction/utils/utils.py
index d126af118..80168f709 100644
--- a/src/easydiffraction/utils/utils.py
+++ b/src/easydiffraction/utils/utils.py
@@ -20,6 +20,7 @@
 import numpy as np
 import pandas as pd
 import pooch
+from filelock import FileLock
 from packaging.version import Version
 from rich.markup import escape
 from uncertainties import UFloat
@@ -440,21 +441,28 @@ def _fetch_data_index() -> dict:
     index_url = _build_data_url('index.json')
     _validate_url(index_url)
 
-    cache_dir = pooch.os_cache('easydiffraction')
+    cache_dir = pathlib.Path(pooch.os_cache('easydiffraction'))
+    cache_dir.mkdir(parents=True, exist_ok=True)
     # Cache under a commit-named file so a ref bump downloads a fresh
     # index instead of reusing a stale one (data-source-pinning ADR).
     destination_fname = f'data-index-{_data_index_ref()}.json'
+    lock_path = cache_dir / f'{destination_fname}.lock'
+
+    # Pooch does not lock ``retrieve`` calls. Parallel processes can
+    # therefore replace the same cache file while another process opens
+    # it, which raises PermissionError on Windows. Keep retrieval and
+    # parsing in one lock.
+    with FileLock(lock_path):
+        index_path = pooch.retrieve(
+            url=index_url,
+            known_hash=None,
+            fname=destination_fname,
+            path=cache_dir,
+            progressbar=False,
+        )
 
-    index_path = pooch.retrieve(
-        url=index_url,
-        known_hash=None,
-        fname=destination_fname,
-        path=cache_dir,
-        progressbar=False,
-    )
-
-    with pathlib.Path(index_path).open('r', encoding='utf-8') as f:
-        return json.load(f)
+        with pathlib.Path(index_path).open('r', encoding='utf-8') as f:
+            return json.load(f)
 
 
 def _existing_project_dir(extraction_dir: pathlib.Path) -> pathlib.Path | None:
diff --git a/tests/unit/easydiffraction/analysis/calculators/test_crysfml.py b/tests/unit/easydiffraction/analysis/calculators/test_crysfml.py
index b7051730e..7fb0d846c 100644
--- a/tests/unit/easydiffraction/analysis/calculators/test_crysfml.py
+++ b/tests/unit/easydiffraction/analysis/calculators/test_crysfml.py
@@ -90,6 +90,16 @@ def test_module_import():
     assert MUT.__name__ == 'easydiffraction.analysis.calculators.crysfml'
 
 
+@pytest.mark.parametrize(
+    ('type_symbol', 'expected'),
+    [('Fe', 'Fe'), ('57Fe', 'Fe'), ('Fe3+', 'Fe'), ('O2-', 'O')],
+)
+def test_element_symbol_strips_isotope_and_ionic_notation(type_symbol, expected):
+    from easydiffraction.analysis.calculators.crysfml import _element_symbol
+
+    assert _element_symbol(type_symbol) == expected
+
+
 def test_crysfml_calculate_pattern_applies_absorption(monkeypatch):
     from easydiffraction.analysis.calculators.crysfml import CrysfmlCalculator
     from easydiffraction.analysis.corrections import absorption
diff --git a/tests/unit/easydiffraction/datablocks/structure/categories/test_atom_sites.py b/tests/unit/easydiffraction/datablocks/structure/categories/test_atom_sites.py
index 10f59dabc..3da6ce03c 100644
--- a/tests/unit/easydiffraction/datablocks/structure/categories/test_atom_sites.py
+++ b/tests/unit/easydiffraction/datablocks/structure/categories/test_atom_sites.py
@@ -77,6 +77,14 @@ def test_type_symbol_setter(self):
         site.type_symbol = 'Fe'
         assert site.type_symbol.value == 'Fe'
 
+    def test_ionic_type_symbol_setter(self):
+        from easydiffraction.datablocks.structure.categories.atom_sites.default import AtomSite
+
+        site = AtomSite()
+        site.type_symbol = 'Fe3+'
+
+        assert site.type_symbol.value == 'Fe3+'
+
     def test_coordinate_setters(self):
         from easydiffraction.datablocks.structure.categories.atom_sites.default import AtomSite
 
diff --git a/tests/unit/easydiffraction/datablocks/structure/item/test_factory.py b/tests/unit/easydiffraction/datablocks/structure/item/test_factory.py
index 148e010ad..0378f1339 100644
--- a/tests/unit/easydiffraction/datablocks/structure/item/test_factory.py
+++ b/tests/unit/easydiffraction/datablocks/structure/item/test_factory.py
@@ -7,3 +7,36 @@
 def test_from_scratch():
     m = StructureFactory.from_scratch(name='abc')
     assert m.name == 'abc'
+
+
+def test_from_cif_str_accepts_underscore_style_structure_tags():
+    cif = """\
+data_legacy
+_cell_length_a 9.15993(5)
+_cell_length_b 9.15993(5)
+_cell_length_c 9.15993(5)
+_cell_angle_alpha 90
+_cell_angle_beta 90
+_cell_angle_gamma 90
+_symmetry_space_group_name_H-M 'P 21 3'
+
+loop_
+_atom_site_label
+_atom_site_type_symbol
+_atom_site_symmetry_multiplicity
+_atom_site_fract_x
+_atom_site_fract_y
+_atom_site_fract_z
+_atom_site_B_iso_or_equiv
+_atom_site_occupancy
+Zr1 Zr4+ 4 0.0003(4) 0.0003(4) 0.0003(4) 0.010(1) 1
+W1  W6+  4 0.3412(3) 0.3412(3) 0.3412(3) 0.012(1) 1
+"""
+
+    structure = StructureFactory.from_cif_str(cif)
+
+    assert structure.cell.length_a.value == 9.15993
+    assert structure.space_group.name_h_m.value == 'P 21 3'
+    assert structure.atom_sites.names == ['Zr1', 'W1']
+    assert structure.atom_sites['Zr1'].type_symbol.value == 'Zr4+'
+    assert structure.atom_sites['Zr1'].multiplicity.value == 4
diff --git a/tests/unit/easydiffraction/io/cif/test_handler.py b/tests/unit/easydiffraction/io/cif/test_handler.py
index 1a7a7e6bc..42b08fb67 100644
--- a/tests/unit/easydiffraction/io/cif/test_handler.py
+++ b/tests/unit/easydiffraction/io/cif/test_handler.py
@@ -58,7 +58,15 @@ def test_cif_read_names_dedup_and_canonical_first():
     handler = TagSpec(edi_names=['_a.x'], cif_names=['_b.y', '_b.z', '_b.y'])
 
     assert handler.cif_name == '_b.y'
-    assert handler.cif_read_names == ['_b.y', '_b.z']
+    assert handler.cif_read_names == ['_b.y', '_b.z', '_b_y', '_b_z']
+
+
+def test_cif_read_names_add_underscore_alias_for_dotted_name():
+    from easydiffraction.io.cif.handler import TagSpec
+
+    handler = TagSpec(edi_names=['_cell.length_a'])
+
+    assert handler.cif_read_names == ['_cell.length_a', '_cell_length_a']
 
 
 def test_cif_names_default_to_edi_names():
@@ -76,4 +84,4 @@ def test_read_names_union_orders_edi_before_cif_and_dedupes():
     handler = TagSpec(edi_names=['_a.x'], cif_names=['_a.x', '_b.y'])
 
     # Edi name first, then CIF-only aliases, with duplicates removed.
-    assert handler.read_names == ['_a.x', '_b.y']
+    assert handler.read_names == ['_a.x', '_b.y', '_a_x', '_b_y']
diff --git a/tests/unit/easydiffraction/utils/test_utils_coverage.py b/tests/unit/easydiffraction/utils/test_utils_coverage.py
index e18671532..596af43a6 100644
--- a/tests/unit/easydiffraction/utils/test_utils_coverage.py
+++ b/tests/unit/easydiffraction/utils/test_utils_coverage.py
@@ -3,6 +3,8 @@
 
 """Supplementary unit tests for easydiffraction.utils.utils — coverage gaps."""
 
+import concurrent.futures
+import threading
 import urllib.request
 
 import numpy as np
@@ -642,6 +644,43 @@ def test_fetch_data_index_reads_cached_json(monkeypatch, tmp_path):
     assert result == {'1': {'path': 'a.xye'}}
 
 
+def test_fetch_data_index_serializes_shared_cache_access(monkeypatch, tmp_path):
+    import json
+
+    import easydiffraction.utils.utils as MUT
+
+    index_file = tmp_path / 'data-index.json'
+    index_file.write_text(json.dumps({'1': {'path': 'a.xye'}}), encoding='utf-8')
+    first_retrieve_entered = threading.Event()
+    release_first_retrieve = threading.Event()
+    state_lock = threading.Lock()
+    active_retrieves = 0
+    max_active_retrieves = 0
+
+    def fake_retrieve(url, known_hash, fname, path, progressbar):
+        nonlocal active_retrieves, max_active_retrieves
+        with state_lock:
+            active_retrieves += 1
+            max_active_retrieves = max(max_active_retrieves, active_retrieves)
+        first_retrieve_entered.set()
+        release_first_retrieve.wait(timeout=1)
+        with state_lock:
+            active_retrieves -= 1
+        return str(index_file)
+
+    monkeypatch.setattr(MUT.pooch, 'os_cache', lambda name: tmp_path)
+    monkeypatch.setattr(MUT.pooch, 'retrieve', fake_retrieve)
+
+    with concurrent.futures.ThreadPoolExecutor(max_workers=4) as executor:
+        futures = [executor.submit(MUT._fetch_data_index) for _ in range(4)]
+        assert first_retrieve_entered.wait(timeout=1)
+        release_first_retrieve.set()
+        results = [future.result(timeout=1) for future in futures]
+
+    assert results == [{'1': {'path': 'a.xye'}}] * 4
+    assert max_active_retrieves == 1
+
+
 # --- _existing_project_dir ----------------------------------------------------